feat(display): blackout rescue — re-enable a disabled panel on undock

Bernardo's question: with "Laptop screen off" active, does the panel
come back when the dock is yanked? VM answer (new maintainer harness
tools/monitor-fallback.nix, softGL desktop): no — Hyprland 0.55.4
leaves the session with zero active outputs and never re-enables a
soft-disabled monitor. rescue_blackout in the display hotplug watcher
now re-enables every disabled output (immediate, retried, idempotent
keyword + toast) whenever a monitorremoved leaves nothing active;
independent of the profile auto-switch and its settings gate.

Sharp edge documented rather than hidden: the zero-output state itself
crashes Hyprland in the VM ~4/5 runs (aquamarine CBackend::dispatchIdle
ABRT within ~0.5s — faster than any userland rescue; a control run
removing the external with the panel ACTIVE survives 5/5, so the state
is the trigger, not headless removal; crash reproduced with the rescue
provably inert, so not caused by it). Rescue ships as defense-in-depth:
it recovers every surviving case, and the worst case on real DRM is
session-to-greeter on a re-lit panel instead of a black brick. Rider
fix: `grep -c .` exits 1 at count zero — the ||-guard on that pipeline
silently disabled the rescue exactly when it mattered.

Verified: hazard V2 (VM reproduces the dead state); control
discriminator 5/5; flake check green. V3 pending: real-dock
undock-while-off in HARDWARE-QUEUE (run with nothing important open;
crash on real DRM = rework the row to mirror or wait for a Hyprland
bump).

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

docs/TESTING.md

@@ -105,6 +105,11 @@ unattended env it uses is in the script, and the same flow works
 interactively from the live terminal. If the desktop comes up unthemed,
 read `/var/log/nomarchy-hm-preactivate.log` on the installed system.
 
+Two full-desktop VM harnesses (softGL Hyprland, too heavy for `checks.*`)
+live next to the scripts: `tools/theme-shot.nix` (themed-desktop
+screenshots) and `tools/monitor-fallback.nix` (undock blackout rescue —
+disabled panel must re-enable when the last active output disappears).
+
 ## 5. VM-specific gotchas
 
 - **No KVM** (e.g. nested without acceleration): everything works but