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>

README.md

@@ -164,6 +164,7 @@ Day-to-day you'll use the shipped shortcuts instead:
 
 ```sh
 sys-update    # nix flake update + system rebuild (BTRFS snapshot first when available)
+sys-rebuild   # system rebuild against the CURRENT lock (config changes only, no update)
 home-update   # home-manager switch (no flake update, no sudo)
 ```
 
@@ -352,6 +353,7 @@ nomarchy-theme-sync bg next            # cycle wallpapers — instant, no rebuil
 nomarchy-theme-sync bg auto            # back to the theme's default wallpaper
 nomarchy-theme-sync get colors.accent
 sys-update                             # update inputs + rebuild the system (snapshots first)
+sys-rebuild                            # rebuild the system, current lock (no update)
 home-update                            # rebuild just the desktop layer
 ```