feat(#76): harvest request-scoped availability and freshness truth

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>

internal/web/live.go

@@ -364,7 +364,15 @@ func cloneLivePositions(in map[string]live.LivePositionData) map[string]live.Liv
 
 // handleLiveState returns the current live data snapshot as JSON.
 func (s *Server) handleLiveState(w http.ResponseWriter, r *http.Request) {
-	writeJSON(w, s.hub.State())
+	state := s.hub.State()
+	freshness := "limited"
+	if state.IsLive {
+		freshness = "live"
+	} else if state.LastSnapshot != nil {
+		freshness = "archive"
+	}
+	markDataResponse(w, "fia", freshness)
+	writeJSON(w, state)
 }
 
 // handleSSEStream is the persistent SSE endpoint for live data.