perf(snapshots): Buffer image swap to eliminate flash during single-view navigation

[NicoHinderling]

Switching between images in single-view mode had a noticeable delay — the old image would disappear immediately, leaving a blank flash while the browser fetched and decoded the new one.

Two fixes:

loading="eager" + decoding="async" — The core Image component defaults to loading="lazy", which is counterproductive for single-view images since they're always in the viewport. Switching to eager removes the lazy-load heuristic delay. decoding="async" keeps the decode non-blocking.

useBufferedImageUrl hook — Instead of swapping the <img> src immediately (which clears the old image), this hook pre-decodes the new image via new Image().decode() in the background and only swaps the URL once it's paint-ready. The old image stays on screen the entire time, so there's no blank gap.

List-view images are unaffected — they correctly use loading="lazy" via snapshotDiffBodies.tsx.

[NicoHinderling]

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• perf(snapshots): Buffer image swap to eliminate flash during single-view navigation #114271 👈 (View in Graphite)
• ref(snapshots): Final UI tweaks #114232
• feat(preprod): Update snapshot header with approval actions #113959
• feat(preprod): Complete the new snapshot viewer toolbar and controls #113994
• feat(preprod): Add view mode toggle between single and list views #113993
• feat(preprod): Add SnapshotListView virtualized list component #113992
• ref(preprod): Replace sidebar disclosure sections with status filter pills #113988
• ref(preprod): Use transparent color sentinel for overlay visibility #113987
• master

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