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-pipeline/src/ops/dehaze.rs

@@ -99,7 +99,7 @@
 //! A minimum over a patch is separable, as a Gaussian is: minimum along x,
 //! then along y. That alone is not enough. The patch is 1% of the shorter edge
 //! — 61 taps across at 4K — and two passes of 61 taps is the arithmetic that
-//! measured 34 ms for clarity and became `docs/technical-debt.md` TD-4.
+//! measured 34 ms for clarity and became `docs/dev/technical-debt.md` TD-4.
 //!
 //! A minimum has a property a Gaussian does not: **erosions compose by adding
 //! their structuring elements**. The minimum over a contiguous run of `d`

core/dr-pipeline/src/ops/local_contrast.rs

@@ -150,7 +150,7 @@
 //! the artefact this control must not have.
 //!
 //! Run at the render size, that measured **34 ms at 4K** — seven times the
-//! entire fused point chain, for one slider — which is `docs/technical-debt.md`
+//! entire fused point chain, for one slider — which is `docs/dev/technical-debt.md`
 //! TD-4 and is what [`Recipe::base_scale`] now answers. The base is computed on
 //! a grid a quarter the size on each axis: a sixteenth of the pixels at a
 //! quarter of the radius.
@@ -271,7 +271,7 @@ impl Band for Coarse {
         threshold: 0.35,
         gain: 1.0,
         midtone_taper: true,
-        // A quarter, which is what `docs/technical-debt.md` TD-4 bought back.
+        // A quarter, which is what `docs/dev/technical-debt.md` TD-4 bought back.
         //
         // σ is 1.2% of the shorter edge — 26 px at 4K — so the base holds no
         // spatial frequency anywhere near the quarter-scale Nyquist of one