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>

docs/ROADMAP.md

@@ -849,6 +849,24 @@ how to override it. Items marked ✓ are shipped.
   your theme / essentials" flow (ties into the branding work).
 
 ## Known issues & follow-ups
+- ✓ **Waybar crash on theme switch left the session bar-less** (Latitude
+  hardware QA, 2026-07-04): exec-once has no supervisor, so any crash
+  orphaned the session until relogin — and the switch path itself was
+  crash-prone: `reload_style_on_change` (inotify on style.css) and
+  theme-sync's SIGUSR2 both fired while the HM symlinks flipped, a
+  double-reload race in waybar's in-place reload. Fix: (1) the bar runs
+  under a `nomarchy-waybar` supervisor (waybar.nix) — ANY exit respawns
+  it, with a crash-loop guard (5 fast exits → critical notification,
+  stop); (2) theme-sync now prefers a clean `pkill -x waybar` when it
+  sees the supervisor (a restart with fresh config+style — no reload
+  code path at all), keeping SIGUSR2 only as the fallback for
+  unsupervised/custom bars; `reload_style_on_change` stays for manual
+  `home-update` restyles. VM-verified on the headless software-GL
+  desktop: SIGKILL → new pid; clean kill → respawn; SIGUSR2 → alive.
+- ✓ **`sys-rebuild`** (Latitude QA request): the no-update twin of
+  `sys-update` — snapshot-first system rebuild against the *current*
+  lock, for config-only changes; `home-update` was already lock-free.
+  README §3/§5 + the motd list all three.
 - ✓ **Yazi TOML parse error on startup:** yazi 26.x made fetchers'
   `group` field required, so `[[plugin.prepend_fetchers]]` failed to parse
   and yazi fell back to presets. Fixed by adding `group = "git"` to both