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/graph.rs

@@ -113,7 +113,7 @@ pub struct EditGraph {
     /// a sidecar comes to name one stock while the shader draws another.
     film: Option<Film>,
     /// TRACES: FR-DEV-8
-    /// The repairs (`docs/spot-removal.md`).
+    /// The repairs (`docs/dev/spot-removal.md`).
     ///
     /// Apart from `ops` for the third time and the same reason: a spot is not
     /// a scalar, and a list of them is not a slider. It sits beside the masks
@@ -122,7 +122,7 @@ pub struct EditGraph {
     /// photograph comes from.
     spots: SpotSet,
     /// The lens corrections that rewrite coordinates: distortion and lateral
-    /// chromatic aberration (`docs/architecture.md` §5.2).
+    /// chromatic aberration (`docs/dev/architecture.md` §5.2).
     ///
     /// Apart from `ops` for the fourth time, and this one is not about shape
     /// but about direction. Every [`Operation`] is a function from colour to