feat(keyboard): per-device layout memory in-flake (no ~/.local/state)

Phase 2 of the in-flake-state philosophy, mirroring night-light: the
nomarchy-keyboard-watch daemon now remembers per-device layouts in the
git-tracked state file (settings.keyboard.devices) instead of
~/.local/state/nomarchy/keyboard-layouts.

- Reads the LIVE working-tree map (theme-sync get → jq) so a pick this
  session is honoured at once; writes instantly with --no-switch (no
  rebuild). Writes the whole map back at the dot-free parent path
  (read-modify-write) so a device name with a dot can't corrupt the
  tool's dotted set-path (Hyprland only sanitises spaces→-).
- Graduation: nomarchy.keyboard.devices now mkDefault-merges entries
  from settings.keyboard.devices, so each remembered device becomes a
  generated Hyprland device{} block on the next rebuild (reproducible,
  hotplug-applied). A hand-written keyboard.devices.<name> still wins;
  once baked, the watcher's is_declared check makes it step back.
- theme.nix seeds settings.keyboard.devices = {} (sparse state still
  evaluates); options/template descriptions updated; ROADMAP marked done.

Eval-verified (graduation + mkDefault precedence) and the set/get
writer round-trips. Still needs the on-hardware hotplug re-verify (the
picker path isn't testable in CI).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home/options.nix

@@ -107,9 +107,12 @@ in
         hasn't been chosen before, it pops a rofi picker (these layouts plus
         the primary nomarchy.keyboard.layout), applies the choice to that
         keyboard only (a per-device kb_layout, so the built-in board is left
-        alone), and remembers it per-device (~/.local/state) — re-applying
-        automatically on later reconnects. The runtime-remember complement to
-        the declarative keyboard.devices; a stateful runtime piece by design.
+        alone), and remembers it in the git-tracked in-flake state
+        (settings.keyboard.devices, not ~/.local/state) — re-applied
+        automatically on later reconnects and across reboots. Each remembered
+        choice graduates into nomarchy.keyboard.devices on the next rebuild
+        (a generated device block). The runtime-remember complement to the
+        declarative keyboard.devices.
       '';
     };
 
@@ -136,7 +139,9 @@ in
         nomarchy.keyboard.layout for that keyboard only — e.g. an external
         keyboard that's physically a different layout than the laptop's
         built-in one. Hyprland applies it automatically whenever that
-        keyboard is connected.
+        keyboard is connected. The interactive watcher
+        (nomarchy.keyboard.layouts) also writes its remembered picks here on
+        the next rebuild, so a runtime choice graduates into reproducible config.
       '';
     };