fix(waybar): drop the bar to the bottom layer so fullscreen covers it

On Hyprland the `top` layer renders above every window, including a real
fullscreen surface — so a browser video gone fullscreen sat *under* the
bar (BACKLOG item 30). Move the bar to `layer: bottom` in the generated
waybar.nix and all four whole-swap jsoncs (parity rule); the exclusive
zone still reserves its space in normal tiling, but a fullscreen window
now covers it. Rationale comment kept in waybar.nix; data jsoncs left
comment-free (they carry none).

Verified: V0 (nix flake check --no-build, green) + V1
(homeConfigurations.nomarchy.activationPackage builds; built
.config/waybar/config shows "layer": "bottom"). V3 queued in
HARDWARE-QUEUE (fullscreen covers bar + returns on exit; floating-overlap
trade-off).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -47,16 +47,6 @@ Likely a `hyprland`/`kanshi`-style profile layer plus a menu surface;
 split into slices before starting. `[blocked:hw]` for the real
 dock/undock + HDMI-audio verification (see HARDWARE-QUEUE.md).
 
-### 30. Waybar overlaps fullscreen video
-(Raised by Bernardo, 2026-07-07.) A fullscreen YouTube video (browser
-fullscreen) leaves Waybar drawn on top of the video instead of being
-covered. Waybar sits on an overlay/top layer that outranks the
-fullscreen window's surface. Fix so a true fullscreen window hides the
-bar — likely a Hyprland `layerrule`/`windowrule` or lowering Waybar's
-layer, or wiring the bar to auto-hide on fullscreen. Verify with
-browser fullscreen (F on YouTube) AND a native fullscreen app; make
-sure the bar returns on exit. Small, agent-doable.
-
 ### 28. Theme UI review — make every surface stunning `[big]`
 (Raised by Bernardo, 2026-07-05 — placed at the head of NEXT; reorder
 freely.) A thorough visual pass over the whole themed UI — Waybar (the