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.

core/dr-catalog/src/schema.rs

@@ -1009,7 +1009,7 @@ CREATE INDEX face_index_model ON face_index(model_id);
 
 const V8: &str = r#"
 -- TRACES: FR-CULL-8 | FR-CULL-9 | FR-CULL-10 | FR-CULL-11 | FR-CULL-12 | NFR-SEC-5
--- People and faces (docs/faces.md, docs/catalog.md §10).
+-- People and faces (docs/dev/faces.md, docs/dev/catalog.md §10).
 --
 -- Everything here is **derived data** except one column. Faces, landmarks,
 -- embeddings, cluster assignments and suggestions are all reproducible by
@@ -1046,7 +1046,7 @@ CREATE TABLE faces (
     landmarks  BLOB NOT NULL,          -- 5 x (x, y) f32, normalised likewise
     detector_confidence REAL NOT NULL,
     embedding  BLOB NOT NULL,          -- 512 x f16; unit length until V14, raw since
-    -- Source pixels across the aligned 112x112 crop (docs/faces.md §7).
+    -- Source pixels across the aligned 112x112 crop (docs/dev/faces.md §7).
     --
     -- Not cosmetic: it is the honest quality signal for the UI, a feature in
     -- the §8 calibration -- FR-CULL-9 names face size as an axis along which an