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/detail.rs

@@ -315,7 +315,7 @@ pub struct DetailPass {
     /// 52 render pixels at 4K — holds no spatial frequency a quarter-scale
     /// grid cannot represent. Computing it at the render size therefore buys
     /// nothing and costs everything: 105 taps over 8.3 M pixels, twice, which
-    /// measured at 34 ms and is where `docs/technical-debt.md` TD-4 came from.
+    /// measured at 34 ms and is where `docs/dev/technical-debt.md` TD-4 came from.
     /// At a quarter it is a sixteenth of the pixels at a quarter of the
     /// radius, and the result is not an approximation of the full-resolution
     /// base — it is the same band-limited function, sampled where it is still
@@ -568,7 +568,7 @@ pub fn compose_detail(
 /// photograph the photographer thinks they are sharpening.
 ///
 /// It also means ARCH §5.2's stage list, which draws spot removal after
-/// texture and clarity, is not what this does — see `docs/spot-removal.md`
+/// texture and clarity, is not what this does — see `docs/dev/spot-removal.md`
 /// §5.1, which is where the disagreement is written down.
 pub fn compose_detail_with(
     ops: &[Box<dyn Operation>],