fix(keyboard): isolate new-keyboard layout to its own device

The interactive new-keyboard picker applied the chosen layout with
`hyprctl switchxkblayout <device> <index>`, which flips the *shared*
layout for every keyboard without its own per-device config — so picking
a layout for an external board dragged the built-in keyboard along, and
the change stuck after unplug (it's global state, not device-bound).
Compounding it, the candidate `keyboard.layouts` pool was merged into
`input.kb_layout`, loading those layouts onto the session keyboard too.

Apply the choice as a per-device `hyprctl keyword device[<name>]:kb_layout`
instead — the runtime twin of the declarative `keyboard.devices` blocks —
so it isolates that one keyboard and never touches the built-in board.
Keep the candidate pool out of `input.kb_layout` (session keyboard carries
only `keyboard.layout`); the pool is for external boards, applied per-device.

Also put `nomarchy-keyboard-watch` on PATH when enabled (it was store-path
only via exec-once, so undiscoverable) for debugging.

Found on the Latitude 5410 hardware sweep; picker itself verified working.

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

modules/home/options.nix

@@ -105,10 +105,11 @@ in
         picker. When non-empty, a small watcher runs in the session: when a
         keyboard connects that isn't covered by nomarchy.keyboard.devices and
         hasn't been chosen before, it pops a rofi picker (these layouts plus
-        the primary nomarchy.keyboard.layout), applies the choice, 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.
+        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.
       '';
     };