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/ci-local.sh

@@ -180,7 +180,7 @@ job_traceability() {
 
     # Compared against the working tree, not against HEAD.
     #
-    # CI asks `git diff --quiet docs/traceability.md` after regenerating, which
+    # CI asks `git diff --quiet docs/dev/traceability.md` after regenerating, which
     # is right there and wrong here: CI starts from a clean checkout, so the
     # only diff it can see is the one regeneration introduced. Locally the file
     # is usually already modified for honest reasons — an uncommitted feature
@@ -191,14 +191,14 @@ job_traceability() {
     step "traceability: matrix is up to date"
     local before
     before="$(mktemp)"
-    cp docs/traceability.md "${before}"
+    cp docs/dev/traceability.md "${before}"
     cargo run -q -p traceability -- report
-    if diff -q "${before}" docs/traceability.md >/dev/null; then
+    if diff -q "${before}" docs/dev/traceability.md >/dev/null; then
         record "traceability/matrix" 0
     else
-        echo "docs/traceability.md was stale; regenerating changed it:"
-        diff --stat "${before}" docs/traceability.md 2>/dev/null \
-            || diff "${before}" docs/traceability.md | head -20
+        echo "docs/dev/traceability.md was stale; regenerating changed it:"
+        diff --stat "${before}" docs/dev/traceability.md 2>/dev/null \
+            || diff "${before}" docs/dev/traceability.md | head -20
         record "traceability/matrix" 1
     fi
     rm -f "${before}"