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.

docker/android/assemble-apk.sh

@@ -258,7 +258,7 @@ fi
 cp "${SO}" "${OUT}/staging/lib/${ABI}/libdarkroom.so"
 cp "${DEX}" "${OUT}/staging/classes.dex"
 
-# The inference runtime (docs/inference.md §3): ONNX Runtime and Qualcomm's
+# The inference runtime (docs/dev/inference.md §3): ONNX Runtime and Qualcomm's
 # Hexagon backend, beside libdarkroom.so so the app finds them in its own
 # native library directory. The build links none of it — the app dlopens
 # `libonnxruntime.so` at launch and runs on tract if it is not there — so an
@@ -278,7 +278,7 @@ else
 fi
 
 # The models. Android has no other route to one — app-private storage is not
-# user-reachable and the in-app fetch is unbuilt (docs/faces.md §2.2a) — so
+# user-reachable and the in-app fetch is unbuilt (docs/dev/faces.md §2.2a) — so
 # they go in the APK and `android_main` unpacks them on first launch. The
 # sources are `models/face/` and `models/scene/`, shared with the Arch package
 # rather than living under this one platform's directory.