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-gpu/examples/frame_budget.rs

@@ -1,6 +1,6 @@
 //! What a frame actually costs — the measurement FR-DSP-2 is waiting on.
 //!
-//! `docs/display-and-extension.md` §2 argues that tiled computation predates
+//! `docs/dev/display-and-extension.md` §2 argues that tiled computation predates
 //! the fused-shader design and may not need to exist: the composer folds every
 //! active operation into **one dispatch over a viewport-sized target**, so the
 //! problem tiles were invented to solve may already be solved. That argument
@@ -28,7 +28,7 @@
 //! the per-frame CPU half is dominated by shader-source assembly, which is
 //! string formatting and is several times slower unoptimised.
 //!
-//! The committed numbers live in `docs/frame-budget.md`. Rerun this and diff
+//! The committed numbers live in `docs/dev/frame-budget.md`. Rerun this and diff
 //! that file; a regression should be a diff rather than somebody's memory.
 //!
 //! # Why the 99th percentile and not the mean

core/dr-gpu/examples/segment.rs

@@ -1,6 +1,6 @@
 //! Segment an image and write the granularity ladder as false-coloured PPMs.
 //!
-//! The whole point of S15 step 2 (docs/segmentation.md §11): look at the
+//! The whole point of S15 step 2 (docs/dev/segmentation.md §11): look at the
 //! ladder and decide whether clicking through it would land on the things a
 //! person means. No amount of design settles that — the pictures do.
 //!