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.

CLAUDE.md

@@ -2,12 +2,12 @@
 
 Notes for anyone — person or agent — changing this code. They record what
 went wrong once and what the fix looked like, so the same shape is not
-written again. Requirements live in `docs/requirements.md`; this file is
+written again. Requirements live in `docs/dev/requirements.md`; this file is
 about habits, not features.
 
 ## Catalog reads: work is proportional to what changed, never to library size
 
-`docs/catalog.md §1` states the rule. These are the ways it was broken on
+`docs/dev/catalog.md §1` states the rule. These are the ways it was broken on
 the Identity screen, found when every confirm click cost half a second on a
 24k-image library (2026-09-19), and what each fix looked like.