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.

.gitea/workflows/traceability-check.yml

@@ -7,7 +7,7 @@ name: Traceability
 # fail its own threshold. Two rules follow, and the extractor's own tests
 # enforce both:
 #
-#   1. Denominators are parsed from docs/requirements.md at run time.
+#   1. Denominators are parsed from docs/dev/requirements.md at run time.
 #   2. Coverage is |traced ∩ defined| / |defined|, never a raw traced count.
 #
 # This job is static analysis of source comments plus markdown parsing, so it
@@ -74,11 +74,11 @@ jobs:
         run: |
           set -e
           cargo run -q -p traceability -- report
-          if ! git diff --quiet docs/traceability.md; then
+          if ! git diff --quiet docs/dev/traceability.md; then
             echo ""
-            echo "docs/traceability.md is out of date."
+            echo "docs/dev/traceability.md is out of date."
             echo "Run: cargo run -p traceability -- report"
-            git diff --stat docs/traceability.md
+            git diff --stat docs/dev/traceability.md
             exit 1
           fi
 
@@ -127,4 +127,4 @@ jobs:
 
       - name: Summary
         if: always()
-        run: head -30 docs/traceability.md || true
+        run: head -30 docs/dev/traceability.md || true