image-fallback-url-retry-in-composition

Retry a record's image at a variant the source guarantees when the high-resolution one is missing, by holding the URL in composition state and swapping it once in onError. Covers making the disk-cache key follow the mutated URL, resetting the state per record, keeping the swap a no-op the second time so it cannot loop, and the invisible cost — everything hanging off onSuccess (a palette, and the page theme derived…