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`