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/examples/face_index.rs

@@ -8,7 +8,7 @@
 //!
 //! # Why this exists beside the button in the Identity screen
 //!
-//! Indexing a real library is hours of work (docs/faces.md §12.2), and the
+//! Indexing a real library is hours of work (docs/dev/faces.md §12.2), and the
 //! cases where that is worth starting — an overnight pass, a fresh import, a
 //! machine left running — are exactly the ones where holding a window open is
 //! the wrong shape. The check half is useful on its own: it is cheap, it
@@ -181,7 +181,7 @@ fn main() {
              \n\
              Face crops must be sampled from a buffer longer than {}px and the\n\
              store's largest tier is {}px, so every image here would be refused.\n\
-             The measurement behind that floor is docs/faces.md §7b.\n\
+             The measurement behind that floor is docs/dev/faces.md §7b.\n\
              \n\
              Index from the app's Identity screen instead: that pass renders at\n\
              native resolution, which is what the crop needs.",