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/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.
 ///