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>

pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py

@@ -204,11 +204,22 @@ def run_switch() -> None:
         notify("Rebuild FAILED — see terminal / journal", urgency="critical")
         die("rebuild failed (state file already updated; fix and re-run)")
     notify("Changes applied ✓")
-    # Waybar runs from Hyprland's exec-once (not a systemd unit HM would restart
-    # on switch), so nudge the running bar to re-read its freshly rebuilt
-    # config/style. SIGUSR2 = reload; harmless no-op if waybar isn't running.
+    # Waybar runs from Hyprland's exec-once (not a systemd unit HM would
+    # restart on switch), so nudge the running bar onto the freshly rebuilt
+    # config/style. Under the nomarchy-waybar supervisor a plain kill IS a
+    # clean restart with the new files — waybar's in-place SIGUSR2 reload
+    # is what crashed the bar on hardware (double-reload race with
+    # reload_style_on_change while the symlinks flip), so prefer the
+    # restart whenever the supervisor is there to catch it; fall back to
+    # SIGUSR2 for unsupervised/custom bars. No-ops if nothing is running.
     if shutil.which("pkill"):
-        subprocess.run(["pkill", "--signal", "SIGUSR2", "-x", "waybar"], check=False)
+        supervised = subprocess.run(
+            ["pgrep", "-f", "nomarchy-waybar"], capture_output=True
+        ).returncode == 0
+        if supervised:
+            subprocess.run(["pkill", "-x", "waybar"], check=False)
+        else:
+            subprocess.run(["pkill", "--signal", "SIGUSR2", "-x", "waybar"], check=False)
 
 
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