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-segment/src/distance.rs

@@ -34,7 +34,7 @@
 //! And doing it here buys two things a shader could not. It is **exactly
 //! deterministic**, which matters because masks reach the sidecar as indices
 //! and a field that varied by vendor would mean a mask meaning one thing on
-//! the desktop and another on the phone (docs/segmentation.md §6, M5). And it
+//! the desktop and another on the phone (docs/dev/segmentation.md §6, M5). And it
 //! is testable against hand-computed distances with no adapter present.
 //!
 //! # The transform

core/dr-segment/src/hierarchy.rs

@@ -40,7 +40,7 @@ pub struct Edge {
 
 /// A partition of the image into labelled regions, plus how they adjoin.
 ///
-/// The shared interface from docs/segmentation.md §2: arm A produces this
+/// The shared interface from docs/dev/segmentation.md §2: arm A produces this
 /// from a watershed, arm B would produce it from a class map, and the
 /// consumers above cannot tell which.
 #[derive(Debug, Clone, PartialEq)]

core/dr-segment/src/lib.rs

@@ -1,5 +1,5 @@
 //! TRACES: FR-DEV-3i
-//! Region segmentation for local masking (S15, docs/segmentation.md).
+//! Region segmentation for local masking (S15, docs/dev/segmentation.md).
 //!
 //! Local adjustments need to know where the image's regions are before they
 //! can snap a mask to one. This crate is that map, and it is deliberately

core/dr-segment/src/prior.rs

@@ -1,6 +1,6 @@
 //! Arm C — semantic instances as a prior over the watershed merge order.
 //!
-//! docs/segmentation.md §5. The spec calls this the expected winner and it is
+//! docs/dev/segmentation.md §5. The spec calls this the expected winner and it is
 //! what ships, for a reason that survives the model turning out to be narrower
 //! than §4 assumed: the two arms fail in *opposite* directions, so each one
 //! covers the other's failure.
@@ -233,7 +233,7 @@ pub fn apply_semantic_prior(
 /// This is the interaction the whole spike exists to enable, and the reason it
 /// returns *region ids* rather than a raster: a mask that is a set of integers
 /// is diffable, mergeable at node level under FR-NC-9, and cheap in a sidecar
-/// (docs/segmentation.md §1). A raster is none of those.
+/// (docs/dev/segmentation.md §1). A raster is none of those.
 ///
 /// The returned ids are sorted, so the same click always produces the same
 /// mask — which is what lets it be a cache key.

core/dr-segment/src/refine.rs

@@ -60,7 +60,7 @@
 //! So the last step is a **marker-based watershed**. The mask is eroded to
 //! give two markers — confidently inside, confidently outside — and the flood
 //! runs in the ribbon left between them, meeting along the most expensive line
-//! it can find. The cost is a sum of terms, as docs/segmentation.md §2 says it
+//! it can find. The cost is a sum of terms, as docs/dev/segmentation.md §2 says it
 //! should be: the photograph's own edges, and the colour model's disagreement.
 //!
 //! Markers are what make this the right shape rather than the watershed §15
@@ -244,7 +244,7 @@ pub struct RefineOptions {
     /// How much the photograph's own edges count against the colour model in
     /// the flood's cost, `0.0..=1.0`.
     ///
-    /// docs/segmentation.md §2 specifies the cost as *a sum of terms* — image
+    /// docs/dev/segmentation.md §2 specifies the cost as *a sum of terms* — image
     /// gradient always available, semantic evidence added when a model is
     /// present — and this is the mix. At one the boundary lands purely on the
     /// strongest edge in the band; at zero purely where the colour verdict
@@ -378,7 +378,7 @@ const MAX_SAMPLES: usize = 20_000;
 /// floating-point comparison is a stopping rule that can differ between
 /// machines, and a mask that differs between machines reaches the sidecar as
 /// indices meaning one thing on the desktop and another on the phone
-/// (docs/segmentation.md §6).
+/// (docs/dev/segmentation.md §6).
 const ITERATIONS: usize = 12;
 
 /// Half-width of the verdict scale, in nats.
@@ -676,7 +676,7 @@ impl Refinement {
     /// fronts meet along the most expensive line in the ribbon — which is the
     /// watershed, and which is where the boundary belongs.
     ///
-    /// The cost is a sum of terms, as docs/segmentation.md §2 says it should
+    /// The cost is a sum of terms, as docs/dev/segmentation.md §2 says it should
     /// be: the photograph's own edges, and the colour model's disagreement.
     /// Neither alone is right. An edge with no colour meaning is a texture,
     /// and a colour change with no edge is a gradient.
@@ -889,7 +889,7 @@ fn neighbours(p: usize, w: usize, h: usize) -> impl Iterator<Item = usize> {
 /// Edge strength over the opponent features, as one byte per pixel.
 ///
 /// Sobel over the same three numbers the colour model is fitted on, rather
-/// than over plain luma — docs/segmentation.md §3 is explicit that a
+/// than over plain luma — docs/dev/segmentation.md §3 is explicit that a
 /// channel-weighted RGB gradient reads a saturated red edge as weaker than it
 /// looks, and a flag against sky is exactly that edge.
 ///

core/dr-segment/src/semantic.rs

@@ -1,4 +1,4 @@
-//! Semantic segmentation — arm B (S15, docs/segmentation.md §4).
+//! Semantic segmentation — arm B (S15, docs/dev/segmentation.md §4).
 //!
 //! Runs a YOLO instance-segmentation graph over a proxy-resolution image and
 //! returns the instances it found: a class, a score, a box, and a soft mask
@@ -209,7 +209,7 @@ const EMBEDDED_MODEL: &[u8] = include_bytes!("../../../models/segment/yolo26n-se
 const EMBEDDED_CLASSES: &str = include_str!("../../../models/segment/yolo26n-seg.classes.json");
 
 /// The bytes of the model that ships with this crate, for whoever compiles
-/// engines ahead of the first request (docs/inference.md §6).
+/// engines ahead of the first request (docs/dev/inference.md §6).
 #[cfg(feature = "embedded-model")]
 pub fn embedded_model_bytes() -> &'static [u8] {
     EMBEDDED_MODEL
@@ -237,7 +237,7 @@ impl SemanticModel {
 
     pub fn from_bytes(bytes: &[u8], classes: Vec<Arc<str>>) -> Result<Self, SegmentError> {
         // The f32 graph on whatever the device's backend is. An int8 form
-        // for the Hexagon waits on docs/inference.md §10 M7 — the mask
+        // for the Hexagon waits on docs/dev/inference.md §10 M7 — the mask
         // boundary has to be measured before it moves.
         let session = dr_inference_engine::open(
             dr_inference_engine::Role::Segmenter,