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.

tools/fix-face-model-shapes.sh

@@ -4,14 +4,14 @@
 #   ./tools/fix-face-model-shapes.sh IN.onnx OUT.onnx --input NAME=1,3,640,640
 #   ./tools/fix-face-model-shapes.sh IN.onnx OUT.onnx --dim NAME=1
 #
-# The two the face pipeline needs, verified 2026-08-26 (docs/faces.md §12 M1):
+# The two the face pipeline needs, verified 2026-08-26 (docs/dev/faces.md §12 M1):
 #
 #   ... det_500m.onnx  scrfd_500m_640.onnx --input input.1=1,3,640,640
 #   ... det_2.5g.onnx  scrfd_2.5g_640.onnx --input input.1=1,3,640,640
 #   ... det_10g.onnx   scrfd_10g_640.onnx  --input input.1=1,3,640,640
 #   ... w600k_mbf.onnx arcface_mbf_b1.onnx --dim None=1
 #
-# And the three eye-state models, verified 2026-09-19 (docs/faces.md §17).
+# And the three eye-state models, verified 2026-09-19 (docs/dev/faces.md §17).
 # The landmark model's batch is the literal "None" like the embedder's; the
 # two classifiers' is a *named* dim_param "batch":
 #
@@ -43,7 +43,7 @@
 # ## Why it is a script and not a build step
 #
 # Same reason as the segmentation export: the model is not a build input
-# (docs/faces.md §2.2 — the weights are never committed, because InsightFace's
+# (docs/dev/faces.md §2.2 — the weights are never committed, because InsightFace's
 # grant is non-commercial). This runs once, wherever the user's model lives,
 # and the app loads the result. It exists so the transformation is reproducible
 # rather than a binary someone once produced and nobody can regenerate.