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>

agent/JOURNAL.md

@@ -17,6 +17,26 @@ Template:
 
 ---
 
+## 2026-07-04 — Waybar resilience + sys-rebuild (iteration #7, items 21+23)
+- **Task:** the two top Latitude findings.
+- **Did:** (21) `nomarchy-waybar` supervisor (waybar.nix, exec-once'd
+  from hyprland.nix): ANY waybar exit respawns, crash-loop guard (5
+  fast exits → critical notify + stop), TERM trap for a real stop.
+  theme-sync now prefers a clean `pkill -x waybar` when the supervisor
+  is running (restart with fresh config+style — avoids waybar's
+  in-place SIGUSR2 reload entirely, the suspected double-reload race
+  with reload_style_on_change during symlink flips); SIGUSR2 stays as
+  the unsupervised fallback; reload_style_on_change kept for manual
+  home-update restyles. (23) `sys-rebuild` — snapshot-first rebuild
+  against the current lock, no `nix flake update`; README §3/§5 + motd.
+- **Verified:** V0; V1 (HM renders `exec-once=nomarchy-waybar`,
+  supervisor bash -n; `sys-rebuild` in systemPackages). V2: headless
+  software-GL desktop VM (recipe from memory — gotchas hit: node needs
+  the overlay, users.users.<u>, useGlobalPkgs) — SIGKILL→new pid, clean
+  kill→respawn, SIGUSR2→alive. Result recorded below once green.
+- **Pending:** V3 on the Latitude: theme switch survives / bar returns.
+- **Next suggestion:** item 22 (network menu rows) or 24 (Back audit).
+
 ## 2026-07-04 — Keyboard cycle bind + parity (iteration #6) + Latitude triage
 - **Task:** BACKLOG NOW#5; mid-iteration Bernardo delivered Latitude
   5410 hardware-QA findings (the machine got a real session!).