feat(hyprland): themed backdrop — misc:background_color from the palette (item 28c)

The compositor backdrop behind a missing or still-loading wallpaper
was hard black; on light themes that flash reads as a glitch (caught
by the 1080p capture harness on summer-day). Now the theme's base
color, so every wallpaper-less moment stays in-palette.

Also from the summer-day capture review: the authored whole-swap menu
+ island bar survive the 28b retunes as one coherent identity (the
slate menu deliberately mirrors the bar strip — an earlier "polarity"
concern retracted); nit filed for the undimmed placeholder in the two
summer rasi. Punch list updated in item 28.

Verified: V0; V1 — template-home rebuilt, hyprland.conf renders
background_color=rgb(1a1b26). Visual confirmation rides the next
capture run.

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

agent/BACKLOG.md

@@ -82,9 +82,20 @@ review:
   - **Semantic dim:** bar dim states are `alpha(@text, 0.5)` tints;
     the palette's `muted` role is now floor-guaranteed (28b) — one
     semantic source, use `@muted`.
+  - ✓ **compositor backdrop** (shipped iteration #35): Hyprland
+    `misc:background_color` now the theme's base — no more black flash
+    behind a missing/slow wallpaper (glaring on light themes in the
+    summer-day capture).
+  - summer-day whole-swap capture reviewed (iteration #35): the
+    authored slate menu + island-pill bar survive the 28b retunes and
+    read as one coherent identity — keep. One nit: its rasi doesn't
+    dim the entry placeholder, so "Menu Type to search…" reads as one
+    run-on phrase (generated theme dims it; mirror that in the two
+    summer rasi).
   - Harness gap: wallpaper needs `~/.nomarchy` seeded in the guest
     (theme-sync reads runtime state there) — add before vibe-review
-    captures; whole-swap captures (summer-day/night) still pending;
+    captures; latte + an identity theme (lumon/retro-82) captures
+    still pending;
 (d) the whole-swap themes brought to the same bar (module parity
 verified textually 2026-07-05: whole-swaps carry every functional
 module; omitting cpu/memory is identity). Visual work is