Backend-only re-cut of the #76 availability work onto main, stacked on the
canonical Weekend Context API. Adds request-scoped freshness reporting so
aggregate responses cannot report fresh when a component is stale, plus
local-first driver summary resolution and cache/pacing truth.
The frontend half of #76 is deliberately excluded: it is built on the
Weekend shell that failed owner review, including the full-width Partial
banner treatment. Availability presentation is re-cut with the shell in #89.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Spike: replay location fan-out is bounded with a semaphore at 4 concurrent GetLocation calls, covered by TestAssembleReplayFramesBoundsLocationFanOut.
Adds /api/v1/replay/frames with 5s interval floor, 3000-frame cap, nearest-sample snapping, and empty-driver omission. Wires RaceStoryCanvas to lazy-load replay frames plus cached track outlines when the map panel opens, sharing the existing scrubber/playback state across chart and map.
Tests: HOME=/private/tmp/box-box-home GOCACHE=/private/tmp/box-box-go-cache GOMODCACHE=/Users/aman/go/pkg/mod go test ./internal/web; go build ./... (same env, passed with read-only module stat-cache warning); npm run test; npx tsc --noEmit. Full go test ./... is blocked in this sandbox by existing network/listener-dependent internal/api and internal/news tests.
