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.

ui/dr-ui/src/segmentation.rs

@@ -6,7 +6,7 @@
 //! # What used to be here
 //!
 //! A watershed over-segmented the image, a merge tree turned that into a
-//! granularity ladder, and a click walked up it (docs/segmentation.md, arms A
+//! granularity ladder, and a click walked up it (docs/dev/segmentation.md, arms A
 //! and C). It is gone from this path, and the reason is measured rather than
 //! aesthetic: on a real photograph the saddles are near zero almost
 //! everywhere, so the merge order joins everything meaningful before it joins
@@ -354,7 +354,7 @@ pub struct Options {
     /// Run the model over overlapping tiles instead of the whole frame once.
     ///
     /// Off by default and deliberately so. The graph's input is fixed at
-    /// 640x640 (docs/segmentation.md F6), so every image is letterboxed into
+    /// 640x640 (docs/dev/segmentation.md F6), so every image is letterboxed into
     /// it and a subject 200px across in a 1600px proxy reaches the model at
     /// 80px — which is where a coarse outline comes from. Tiling is the only
     /// route to more resolution with a fixed window, and it costs one