feat(themes): #62 whole-swap CSS self-containment CI gate

Add tools/check-theme-wholeswap.py and checks.theme-wholeswap: every
themes/*/waybar.css must @define-color each @name it references (raw
readFile, no palette prepend — neon-glass class), and every preset needs
preview.png + non-empty backgrounds/. README: four waybar.css themes
(drop quarantined neon-glass). Close #62.

Verified: V0 (script + negative fixture + checks.theme-wholeswap + flake check --no-build).

agent/BACKLOG.md

@@ -113,12 +113,6 @@ gating. Fix: name-agnostic scan aligned with battery-notify. Cost:
 small multi-file; `[blocked:hw]` only to confirm on non-BAT* hardware —
 logic still ships at V1.
 
-### 62. Optional CI: whole-swap CSS self-containment
-Contrast check does not require that a `waybar.css` defines every
-`@color` it references, nor that every preset has preview + backgrounds.
-A cheap gate would have caught neon-glass. Cost: small tool + flake
-check; V0.
-
 ### 63. Generated Waybar: identity-bar affordances (powermenu + logo)
 Whole-swap bars ship `custom/powermenu` and a left logo → main menu; the
 generated `waybar.nix` bar has neither. Optional parity add for
@@ -238,7 +232,7 @@ _(Generated Waybar missing identity-bar affordances → **#63**.)_
   executive-slate, and the generated style have. Functional OK;
   visual hierarchy weak. Cost: CSS pass; V3.
 
-_(Optional CI: whole-swap CSS self-containment → **#62**.)_
+_(Optional CI: whole-swap CSS self-containment → **#62** ✓ 2026-07-10.)_
 
 ### Per-theme design audit (2026-07-09)