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.

packaging/windows/darkroom.nsi

@@ -1,6 +1,6 @@
 ; DarkRoom — Windows installer
 ;
-; Built by docker/windows/package.sh with makensis on Linux; see docs/windows.md
+; Built by docker/windows/package.sh with makensis on Linux; see docs/dev/windows.md
 ; §5 for every decision below. Nothing here is Windows-specific to *write* —
 ; NSIS has always compiled on POSIX hosts.
 ;
@@ -34,7 +34,7 @@ SetCompressor /SOLID lzma
 Name "${NAME} ${VERSION}"
 OutFile "${OUT}"
 
-; Per-user, no elevation (docs/windows.md §5.1). An unsigned installer that
+; Per-user, no elevation (docs/dev/windows.md §5.1). An unsigned installer that
 ; also asks for administrator rights is the most alarming thing Windows can
 ; show, and a per-user install keeps the binaries under the same account as
 ; the library the application builds.
@@ -105,7 +105,7 @@ FunctionEnd
 
 Section "Uninstall"
   ; What the installer wrote, and nothing the application wrote
-  ; (docs/windows.md §5.3): %LOCALAPPDATA%\darkroom and %APPDATA%\darkroom —
+  ; (docs/dev/windows.md §5.3): %LOCALAPPDATA%\darkroom and %APPDATA%\darkroom —
   ; the catalog, the thumbnails, the face index, the settings — stay. An
   ; uninstaller that deletes a library index the user spent two hours
   ; building is the destructive default this project argues against