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-inference-engine/Cargo.toml

@@ -6,7 +6,7 @@ rust-version.workspace = true
 license.workspace = true
 
 # The one crate that names a runtime, a provider, a vendor library or a
-# device (docs/inference.md §8). `dr-face` and `dr-segment` ask it for a
+# device (docs/dev/inference.md §8). `dr-face` and `dr-segment` ask it for a
 # session by role and never see which of these answered.
 
 [dependencies]