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/fill.rs

@@ -95,7 +95,7 @@ pub struct Params {
     /// and, beyond the band being filled, still unknown. That is what the
     /// shipped model was trained on (a fine-tune of MI-GAN on voids cut
     /// from photographs the way a cylindrical merge cuts them, see
-    /// `docs/panorama.md` §14); a ring would give it a fold to continue.
+    /// `docs/dev/panorama.md` §14); a ring would give it a fold to continue.
     ///
     /// Non-zero is the stock model's crutch: a plain reflection of a deep
     /// hole pulls in whatever is that far from the edge — a ridge, a peak —

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

core/dr-pano/src/xfeat.rs

@@ -4,7 +4,7 @@
 //! Apache-2.0 weights (`models/LICENCE.md`), exported at a fixed shape by
 //! `tools/export-xfeat.sh` and loaded through the same `dr-inference-engine`
 //! `dr-segment` and `dr-face` use, so this adds no runtime and no C to the
-//! tree; what runs it is the device's business (docs/inference.md). ~300 ms
+//! tree; what runs it is the device's business (docs/dev/inference.md). ~300 ms
 //! per frame on tract on the reference desktop, ~400 ms on the tablet
 //! (S15.2, S15.4).
 
@@ -37,7 +37,7 @@ pub struct XFeat {
 }
 
 /// The bytes of both exports compiled into the binary, for whoever compiles
-/// engines ahead of the first request (docs/inference.md §6).
+/// engines ahead of the first request (docs/dev/inference.md §6).
 #[cfg(feature = "embedded-model")]
 pub fn embedded_model_bytes() -> [&'static [u8]; 2] {
     [EMBEDDED_LANDSCAPE, EMBEDDED_PORTRAIT]