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-pano/src/migan.rs

@@ -9,7 +9,7 @@
 //! on every rung, and what they cost is the whole story of whether a fill
 //! is interactive: 7.4 s a tile under tract, 0.4 s under ONNX Runtime's
 //! CPU pool, 23 ms in fp16 and 13 ms in int8 on a laptop's TensorRT
-//! (2026-09-19, docs/panorama.md §12).
+//! (2026-09-19, docs/dev/panorama.md §12).
 //!
 //! The model's contract, from the reference `export_inference_model.py`:
 //! input `1×4×512×512` float — channel 0 is `mask − 0.5` with 1 where the