fix(waybar): supervisor respawn + clean-restart switches; add sys-rebuild

Recovers the crashed iteration #7 (items 21+23, Latitude QA findings):

- nomarchy-waybar supervisor (waybar.nix, exec-once'd from
  hyprland.nix): any waybar exit respawns the bar; crash-loop guard
  (5 fast exits -> critical notify + stop); TERM trapped for a real
  stop. Fixes the bar-less session after a theme-switch crash.
- nomarchy-theme-sync prefers a clean `pkill -x waybar` (supervisor
  restart with fresh config+style) over the crash-prone in-place
  SIGUSR2 reload when the supervisor is running; SIGUSR2 stays as the
  unsupervised fallback.
- sys-rebuild: snapshot-first system rebuild against the CURRENT lock
  (no `nix flake update`) — the no-update twin of sys-update;
  README §3/§5 + motd list it.

Verified: V0 re-run green after crash recovery (flake check --no-build
+ py_compile); V1/V2 per the crashed session's journal entry (HM
renders exec-once=nomarchy-waybar; headless software-GL VM:
SIGKILL -> new pid, clean kill -> respawn, SIGUSR2 -> alive).
V3 pending on the Latitude (queued in HARDWARE-QUEUE.md).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/hyprland.nix

@@ -209,9 +209,10 @@ in
         # Waybar is launched here rather than as a systemd user service: bound
         # to graphical-session.target the unit raced Hyprland's IPC on relogin
         # and landed in `failed`; exec-once only fires once the compositor is
-        # up. Reloaded on theme switch via SIGUSR2 (nomarchy-theme-sync). See
-        # waybar.nix.
-      ] ++ lib.optional config.nomarchy.waybar.enable "waybar"
+        # up. Via the nomarchy-waybar supervisor (waybar.nix): a crash — or
+        # the clean pkill a theme switch now does — respawns the bar instead
+        # of orphaning the session bar-less.
+      ] ++ lib.optional config.nomarchy.waybar.enable "nomarchy-waybar"
         ++ lib.optional kbAutoSwitch "${keyboardWatch}/bin/nomarchy-keyboard-watch";
 
       # ── Theme-driven look ──────────────────────────────────────────