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.

docker/windows/Dockerfile

@@ -1,6 +1,6 @@
 # DarkRoom — reproducible Windows cross-build environment
 #
-# Everything docs/windows.md §2 names: Rust with the GNU Windows target, the
+# Everything docs/dev/windows.md §2 names: Rust with the GNU Windows target, the
 # MinGW-w64 cross compiler it links with, NSIS to build the installer, and Wine
 # to smoke-test the result. Both CI and local builds use this image, so "works
 # on my machine" and "works in CI" are the same machine — the same argument
@@ -77,7 +77,7 @@ RUN curl -fsSL https://sh.rustup.rs | sh -s -- \
 # libwinpthread the Rust target's own MinGW pieces were built against, and
 # picking the other produces link errors that read as if std were missing.
 #
-# The runtime is linked statically (docs/windows.md §2) so the installer
+# The runtime is linked statically (docs/dev/windows.md §2) so the installer
 # carries one file. `-static-libgcc` is all it takes: rustc's windows-gnu
 # target links its own copy of winpthread in self-contained mode, so nothing
 # imports libwinpthread-1.dll — the smoke test's objdump step is what checks