fix: avoid duplicate world map background loads

.superpowers/sdd/2026-08-18-first-visible-vertical-slice/task-9-report.md

@@ -31,6 +31,7 @@ The reference screenshots were inspected but are not shipped or referenced by th
 - The decision panel now occupies normal document flow below the map scene, reserving a non-overlapping zone at desktop and narrow breakpoints.
 - The original delivered PNGs remain untouched. A deterministic local System.Drawing JPEG conversion (quality 86, high-quality bicubic) provides runtime derivatives: `map_ashen_realm-1440.jpg` (1440x810, 233,778 B), `Suedtor-960.jpg` (960x540, 94,286 B), and `Aschestrasse-960.jpg` (960x540, 93,277 B). The WorldPage and ContextPanel prefer these derivatives while retaining each server artwork path as the `<img>` fallback.
 - The load error is exposed as an alert and its retry control is covered by a store-load regression test.
+- The map no longer declares the runtime JPEG and original PNG as comma-separated background layers. It uses the original PNG as the classic CSS fallback and, where `image-set` with JPEG is supported, replaces it with one runtime JPEG candidate. The WorldPage regression test verifies there is no inline multi-layer binding or `mapBackground` source left to reintroduce the double load.
 
 ## Verification
 
@@ -43,6 +44,7 @@ The reference screenshots were inspected but are not shipped or referenced by th
 - Review full web suite: 23/23 pass.
 - Affected API seed test: 2/2 pass.
 - Both review builds (`npm run build:web`, `npm run build:api`) pass.
+- Review-fix round 2: focused WorldPage test 7/7 pass; full web suite and web build re-run after the fallback change.
 
 ## Remaining concern