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-pipeline/src/mask.rs

@@ -27,7 +27,7 @@
 //! [`MaskSource::Regions`] stores integers naming regions in the segmentation
 //! hierarchy (`dr-segment`). That choice is what makes a mask diffable, cheap
 //! in a sidecar, and mergeable per-field under FR-NC-9 — three properties a
-//! stored raster has none of (docs/segmentation.md §1). Two devices that
+//! stored raster has none of (docs/dev/segmentation.md §1). Two devices that
 //! select the same subject produce the same small sorted list, and a sync
 //! conflict between them is resolvable rather than a binary blob fight.
 //!
@@ -564,7 +564,7 @@ pub enum MaskSource {
     /// This is what the watershed and the semantic model exist to produce.
     /// Selecting a subject means "the regions the model's instance covers",
     /// and the resulting edge is the watershed's, which is to say the image's
-    /// own (docs/segmentation.md §5).
+    /// own (docs/dev/segmentation.md §5).
     Regions {
         /// Which segmentation these ids index into.
         ///
@@ -590,7 +590,7 @@ pub enum MaskSource {
     /// **The primary way a local adjustment is made.** The watershed hierarchy
     /// this crate was first built around does not survive a photograph: its
     /// saddles are near zero almost everywhere, so a global cut collapses the
-    /// frame into one region plus noise (docs/segmentation.md §15). A model
+    /// frame into one region plus noise (docs/dev/segmentation.md §15). A model
     /// instance is a whole object, found as one thing, and needs no ladder.
     ///
     /// The trade is that the boundary is the model's — a quarter-resolution
@@ -625,7 +625,7 @@ pub enum MaskSource {
     /// reason both exist. A subject is *one* instance — this dog, not that one
     /// — found by a COCO-trained instance model. A category is *all* the sky,
     /// or all the foliage, from an ADE20K-trained semantic model that has no
-    /// notion of instances at all (docs/segmentation.md §16).
+    /// notion of instances at all (docs/dev/segmentation.md §16).
     ///
     /// So this is what a global grade attaches to: lift the sky, desaturate
     /// the vegetation, warm the architecture. Asking it for "that person
@@ -2231,7 +2231,7 @@ fn reveal_block(slot: usize, layer: &MaskLayer, style: RevealStyle, colour: [f32
 /// **not** a hash of the label field: that would be a readback on a path that
 /// must not have one (ARCH §6.1), and would also make the signature depend on
 /// float arithmetic whose cross-vendor determinism is exactly the open
-/// question (docs/segmentation.md §6, M5).
+/// question (docs/dev/segmentation.md §6, M5).
 pub fn segmentation_signature(width: u32, height: u32, regions: u32, tuning: u64) -> u64 {
     // FNV-1a over the four fields. Small, dependency-free, and adequate: this
     // guards against accidental mismatch, not against a forged sidecar.