Put the developer docs under docs/dev and index the folder for users first

docs/ had 26 developer documents flat beside the manual, and the two
audiences are very differently sized: most readers want the manual and
the gesture reference, a few want the register, the designs and the
measurements. The manual and gestures.md stay at the top; everything for
someone changing the code moves to docs/dev/, and the two documents that
name their own successors — the v0.1 milestone and the UI-refinement plan
— go to docs/dev/archive/ rather than being deleted, since both are still
cited. docs/README.md is the index, users first.

Every reference follows: code comments, Cargo manifests, the workflows,
the pre-commit hook, the bench and traceability tools (which locate the
repo root by docs/dev/requirements.md now), packaging, the Docker READMEs,
CLAUDE.md, CONTRIBUTING.md and the README. The matrix links one level
deeper and is regenerated. Links out of the moved documents into the tree
gain a level; a link checker over every Markdown file finds none broken.

core/dr-face/src/align.rs

@@ -1,4 +1,4 @@
-//! Five-point face alignment (docs/faces.md §5).
+//! Five-point face alignment (docs/dev/faces.md §5).
 //!
 //! ArcFace embeddings are trained on faces warped to a canonical 112×112
 //! arrangement. Feeding the model a plain bounding-box crop *works* — it
@@ -208,7 +208,7 @@ impl Similarity {
 ///
 /// # Why least squares and not RANSAC
 ///
-/// The reference C++ implementation (docs/faces.md §1.1) fits this with
+/// The reference C++ implementation (docs/dev/faces.md §1.1) fits this with
 /// OpenCV's `estimateAffinePartial2D` under RANSAC. RANSAC over five points is
 /// a strange fit: the minimal sample for a similarity is two, so it can discard
 /// landmarks it judges outliers and solve from a subset — and on a profile face
@@ -427,7 +427,7 @@ fn sample_window(
 // ── eyes ──────────────────────────────────────────────────────────────────
 
 /// Width of an eye crop as the classifier reads it, in pixels. Fixed by the
-/// OCEC input (`docs/faces.md` §17): 40 wide, 24 high.
+/// OCEC input (`docs/dev/faces.md` §17): 40 wide, 24 high.
 pub const EYE_PATCH_WIDTH: usize = 40;
 /// Height of an eye crop as the classifier reads it, in pixels.
 pub const EYE_PATCH_HEIGHT: usize = 24;
@@ -438,7 +438,7 @@ pub const EYE_PATCH_HEIGHT: usize = 24;
 /// The classifier was trained on a whole-body detector's *eye* boxes — tight
 /// round the palpebral fissure — and measured on 25 open-eyed faces from the
 /// reference library, a tight box is what it wants: 22 of 25 read open at
-/// 0 and 0.1, 18 at 0.4, 14 at 0.6 (docs/faces.md §17.2). A tenth, so a
+/// 0 and 0.1, 18 at 0.4, 14 at 0.6 (docs/dev/faces.md §17.2). A tenth, so a
 /// contour landing a pixel short of the lashes still holds them.
 pub const EYE_BOX_MARGIN: f32 = 0.1;
 
@@ -571,7 +571,7 @@ pub const SUNGLASSES_EDGE: usize = 48;
 /// clear glasses, at 0.68. Erring towards "sunglasses" is the safe direction
 /// for what this feeds: a face called sunglasses is left alone by the
 /// eyes-open filter, where a pair of sunglasses missed hands the eye
-/// classifier a lens to guess at (docs/faces.md §17).
+/// classifier a lens to guess at (docs/dev/faces.md §17).
 pub const SUNGLASSES_WINDOWS: [(f32, f32, f32, f32); 2] =
     [(0.0, 0.0, 112.0, 112.0), (-5.0, -14.0, 122.0, 122.0)];
 

core/dr-face/src/calibrate.rs

@@ -1,4 +1,4 @@
-//! Cosine to probability (docs/faces.md §8, FR-CULL-9).
+//! Cosine to probability (docs/dev/faces.md §8, FR-CULL-9).
 //!
 //! FR-CULL-9 is a hard requirement rather than an implementation detail: no
 //! code path may threshold a bare cosine, every threshold in the subsystem is
@@ -28,7 +28,7 @@
 //! calibration to the belief it was supposed to test — and that is the whole
 //! of the alternative.
 //!
-//! docs/faces.md §8.1 names one more that would cost no labelling at all: two
+//! docs/dev/faces.md §8.1 names one more that would cost no labelling at all: two
 //! faces in adjacent frames of one burst are near-certainly the same person,
 //! and FR-CULL-5's grouping is sitting there. Nothing draws on it. This crate
 //! cannot see a catalog, let alone the bursts in one — it is handed cosines by
@@ -83,7 +83,7 @@ pub struct Calibration {
 }
 
 impl Default for Calibration {
-    /// The reference implementation's fitted MBF curve (docs/faces.md §1):
+    /// The reference implementation's fitted MBF curve (docs/dev/faces.md §1):
     /// steepness 16.2, P=0.5 at cosine 0.267.
     ///
     /// **`valid` is false**, and that is the point. It is a documented

core/dr-face/src/classify.rs

@@ -1,5 +1,5 @@
 //! TRACES: FR-CULL-8a
-//! The two small classifiers behind a face's eye state (docs/faces.md §17).
+//! The two small classifiers behind a face's eye state (docs/dev/faces.md §17).
 //!
 //! **OCEC** — *open closed eyes classification*, Hyodo 2025 — reads one
 //! 40×24 eye and answers P(open). **SGC** — *sunglasses classification*,

core/dr-face/src/cluster.rs

@@ -1,4 +1,4 @@
-//! Grouping faces into people (docs/faces.md §9, FR-CULL-10).
+//! Grouping faces into people (docs/dev/faces.md §9, FR-CULL-10).
 //!
 //! Model-free: this is arithmetic over embeddings, and it is where the
 //! subsystem's accuracy actually lives, so it is testable with no weights on

core/dr-face/src/detect.rs

@@ -1,4 +1,4 @@
-//! SCRFD face detection (docs/faces.md §4).
+//! SCRFD face detection (docs/dev/faces.md §4).
 //!
 //! One forward pass produces a box, a confidence and **five landmarks** per
 //! face — the landmarks being the reason for this detector rather than a
@@ -138,7 +138,7 @@ impl Detection {
 pub struct Detector {
     session: Model,
     /// f32 or int8 — the int8 form finds a different set of faces and is a
-    /// different detector in `model_id` (docs/inference.md §7).
+    /// different detector in `model_id` (docs/dev/inference.md §7).
     form: Form,
     /// Feature-map count: 3 for strides {8,16,32}, 4 for {8,16,32,64}.
     ///
@@ -346,7 +346,7 @@ fn iou(a: &(f32, f32, f32, f32), b: &(f32, f32, f32, f32)) -> f32 {
 /// How the image is fitted into the graph's fixed square input.
 ///
 /// The forward and inverse mappings live in one struct on purpose:
-/// docs/faces.md §4.1 notes that what matters is not *where* the padding goes
+/// docs/dev/faces.md §4.1 notes that what matters is not *where* the padding goes
 /// but that the two agree. A mismatch offsets every box and landmark by the
 /// padding, producing detections that look plausible and embeddings that
 /// quietly cluster badly three stages later.
@@ -372,7 +372,7 @@ impl Letterbox {
     ///
     /// `(x·255 − 127.5) / 128` — note `/128`, not `/127.5`. The reference
     /// implementation this is ported from uses `/128` for both models, and
-    /// every measured number in docs/faces.md §1 came from it.
+    /// every measured number in docs/dev/faces.md §1 came from it.
     ///
     /// Padding is grey, matching the reference's `114`: the value the network
     /// reads least as an edge, where black would draw a hard border across the

core/dr-face/src/embed.rs

@@ -1,4 +1,4 @@
-//! ArcFace / MobileFaceNet inference (docs/faces.md §6).
+//! ArcFace / MobileFaceNet inference (docs/dev/faces.md §6).
 //!
 //! Takes an aligned crop and returns 512 L2-normalised floats. The alignment is
 //! not optional and cannot be skipped by accident: [`Embedder::embed`] takes an
@@ -66,7 +66,7 @@ impl Embedder {
 
     pub fn from_bytes(bytes: &[u8], model: ModelId) -> Result<Self, FaceError> {
         // Always the f32 form: an embedding must compare across devices
-        // (docs/inference.md §7), and the engine pins this role to it.
+        // (docs/dev/inference.md §7), and the engine pins this role to it.
         let loaded = dr_inference_engine::open(Role::Embedder, Form::F32, bytes)?;
         let acquired = loaded.acquire()?;
         let session = acquired.lock();

core/dr-face/src/embedding.rs

@@ -1,4 +1,4 @@
-//! What an embedder produces, and how it is stored (docs/faces.md §6).
+//! What an embedder produces, and how it is stored (docs/dev/faces.md §6).
 //!
 //! Deliberately **model-free**: the vector, its identity, its comparison and
 //! its storage encoding are arithmetic, and `calibrate` and `cluster` are built
@@ -273,7 +273,7 @@ mod tests {
         );
     }
 
-    /// The claim docs/faces.md §6 makes about the storage format: the f16
+    /// The claim docs/dev/faces.md §6 makes about the storage format: the f16
     /// round-trip costs ~1e-3 of cosine, three orders below the separation
     /// between a match and a non-match.
     #[test]

core/dr-face/src/eyes.rs

@@ -67,14 +67,14 @@ pub const SUNGLASSES_THRESHOLD: f32 = 0.5;
 /// The classifier was trained on eyes down to about a dozen pixels wide
 /// (its reference footage averaged 15–21); below that the 40-pixel patch is
 /// an interpolation of nothing, and the answer is noise that reads as
-/// "closed". docs/faces.md §17.3 has the measurement behind the number.
+/// "closed". docs/dev/faces.md §17.3 has the measurement behind the number.
 pub const MIN_EYE_PX: f32 = 12.0;
 
 /// Least [`Eye::sharpness`] for the eye to be read.
 ///
 /// The same measure as the face's `min_sharpness`, over the eye patch, and
 /// chosen the same way: the value under which the open-eyed faces of the
-/// reference sample were being called closed. docs/faces.md §17.3.
+/// reference sample were being called closed. docs/dev/faces.md §17.3.
 pub const MIN_EYE_SHARPNESS: f32 = 0.02;
 
 /// An eye narrower than this fraction of its partner is the far eye of a
@@ -82,7 +82,7 @@ pub const MIN_EYE_SHARPNESS: f32 = 0.02;
 ///
 /// A landmark model's contour for a hidden eye collapses towards the nose.
 /// Measured on twenty native renders of the reference library
-/// (docs/faces.md §17.4): profiles put the far eye at 0.02–0.43 of the near
+/// (docs/dev/faces.md §17.4): profiles put the far eye at 0.02–0.43 of the near
 /// one, two three-quarter faces whose far eye read closed sat at 0.54, and
 /// every face looking at the camera — winks included, since a shut eye's
 /// box keeps its width — sat at 0.78 or more. 0.6 splits the gap.

core/dr-face/src/landmarks.rs

@@ -1,5 +1,5 @@
 //! TRACES: FR-CULL-8a
-//! Dense facial landmarks — InsightFace's `2d106det` (docs/faces.md §17.2).
+//! Dense facial landmarks — InsightFace's `2d106det` (docs/dev/faces.md §17.2).
 //!
 //! SCRFD's five points place a face; they do not place an eye. Its eye
 //! point is loose enough that a window centred on it left the eye in a

core/dr-face/src/lib.rs

@@ -1,4 +1,4 @@
-//! Faces and identity (S14, docs/faces.md).
+//! Faces and identity (S14, docs/dev/faces.md).
 //!
 //! Two models, run over the native render, producing per face a box, five
 //! landmarks, a confidence and a 512-d embedding (FR-CULL-8) — and then the
@@ -21,9 +21,9 @@
 //! for a packaging script to switch on. The application obtains a model at
 //! runtime; this crate takes bytes and never fetches anything.
 //!
-//! docs/faces.md §2 is the full reading, including what would have to change
+//! docs/dev/faces.md §2 is the full reading, including what would have to change
 //! for that to stop being true. The eye-state models are the exception: MIT,
-//! weights and all, and shipped in `models/face/` (docs/faces.md §17).
+//! weights and all, and shipped in `models/face/` (docs/dev/faces.md §17).
 //!
 //! # Why the runtime is split behind a feature
 //!
@@ -55,7 +55,7 @@ pub mod references;
 /// Smallest long edge a face crop may be sampled from.
 ///
 /// **A floor on the crop source, not on the detector input.** The distinction
-/// is the whole of FR-CULL-8 and `docs/faces.md` §7: detection letterboxes
+/// is the whole of FR-CULL-8 and `docs/dev/faces.md` §7: detection letterboxes
 /// every buffer into 640×640, so its input resolution decides nothing, while
 /// [`warp`] samples the 112×112 the embedder sees and so converts source
 /// resolution directly into embedding quality. FR-CULL-8 requires that crop to

core/dr-face/src/neighbours.rs

@@ -115,7 +115,7 @@ pub struct Faces<'a> {
     /// Source pixels across the aligned crop, for the calibration's size term.
     pub crop_px: &'a [f32],
     /// Which photograph each face came from. Two faces in one frame are not
-    /// the same person, so those pairs are never returned (docs/faces.md §9).
+    /// the same person, so those pairs are never returned (docs/dev/faces.md §9).
     pub images: &'a [u64],
     /// Which faces may be compared *against* — the gallery
     /// ([`crate::embedding::MIN_GALLERY_QUALITY`]).