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/spot.rs

@@ -6,7 +6,7 @@
 //! are blended. No pixels are stored, here or anywhere: the shader draws the
 //! repair from these numbers every time the photograph is rendered, which is
 //! what makes it non-destructive, cheap to sync, and undoable
-//! (`docs/spot-removal.md`).
+//! (`docs/dev/spot-removal.md`).
 //!
 //! # Why this is not an operation
 //!
@@ -61,7 +61,7 @@ pub const MAX_SPOTS: usize = 64;
 /// bounds something that is otherwise unbounded: a detail pass declares how far
 /// it reads from the pixel it writes, and for a spot that is the offset plus
 /// the radius. An unbounded offset is an unbounded halo, which is a pass the
-/// tile scheduler cannot plan (ARCH §5.3, `docs/spot-removal.md` §5.3).
+/// tile scheduler cannot plan (ARCH §5.3, `docs/dev/spot-removal.md` §5.3).
 pub const MAX_SOURCE_DISTANCE: f32 = 0.5;
 
 /// The radius a new spot starts at, in frame units.
@@ -210,7 +210,7 @@ impl Spot {
     /// **FR-DEV-8 asks for automatic source placement, and this is the cheap
     /// half of it.** The good half searches the photograph for a patch whose
     /// surroundings match — a compute dispatch scoring candidate offsets, and
-    /// one small readback when the spot is created (`docs/spot-removal.md`
+    /// one small readback when the spot is created (`docs/dev/spot-removal.md`
     /// §8). This is what stands in for it, and it is worth having on its own
     /// terms rather than as a placeholder: dust sits on skies, skies are
     /// smooth, and a patch two and a half radii away is nearly always the same