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/develop.rs

@@ -258,7 +258,7 @@ impl SegmentationJob {
     ///
     /// The model reads the photograph as captured, not as edited: the
     /// segmentation must survive an exposure change, or every slider would
-    /// invalidate the masks that depend on it (docs/segmentation.md §3).
+    /// invalidate the masks that depend on it (docs/dev/segmentation.md §3).
     ///
     /// **Still the sensor's orientation, deliberately.** Standing the picture
     /// up is what `segmentation::compute` does to the buffer this returns,
@@ -2387,7 +2387,7 @@ impl DevelopSession {
     }
 
     // ----------------------------------------------------------------------
-    // Segmentation (S15, docs/segmentation.md)
+    // Segmentation (S15, docs/dev/segmentation.md)
     // ----------------------------------------------------------------------
 
     /// This session's name, carried by any work started against it.
@@ -3569,7 +3569,7 @@ impl DevelopSession {
     /// The instance's own coverage is the mask, rather than the watershed
     /// regions it overlaps. Snapping to regions was the original design and
     /// it is not currently worth doing: the hierarchy those ids index into
-    /// collapses on a photograph (docs/segmentation.md §15), so snapping
+    /// collapses on a photograph (docs/dev/segmentation.md §15), so snapping
     /// would trade the model's approximately-right outline for a
     /// confidently-wrong one.
     pub fn add_subject_mask(&mut self, index: usize) -> Option<String> {