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/freshness.go

@@ -0,0 +1,65 @@
+package web
+
+import (
+	"net/http"
+)
+
+const (
+	dataSourceHeader    = "X-BoxBox-Data-Source"
+	dataFreshnessHeader = "X-BoxBox-Data-Freshness"
+)
+
+type staleResponseReporter interface {
+	LastResponseWasStale() bool
+}
+
+// markOpenF1Response publishes request-scoped success provenance. Callers must
+// pass the scoped client used for this response, never Server.client.
+func markOpenF1Response(w http.ResponseWriter, client staleResponseReporter) {
+	markOpenF1AggregateResponse(w, client, false)
+}
+
+func markOpenF1AggregateResponse(w http.ResponseWriter, client staleResponseReporter, partial bool) {
+	freshness := "fresh"
+	if partial {
+		freshness = "partial"
+	}
+	markOpenF1Availability(w, client, freshness)
+}
+
+func markOpenF1Availability(w http.ResponseWriter, client staleResponseReporter, freshness string) {
+	w.Header().Set(dataSourceHeader, "openf1")
+	if client != nil && client.LastResponseWasStale() {
+		w.Header().Set(dataFreshnessHeader, "stale")
+		return
+	}
+	if freshness == "" {
+		freshness = "fresh"
+	}
+	w.Header().Set(dataFreshnessHeader, freshness)
+}
+
+func markDataResponse(w http.ResponseWriter, source, freshness string) {
+	w.Header().Set(dataSourceHeader, source)
+	w.Header().Set(dataFreshnessHeader, freshness)
+}
+
+func markLocalResponse(w http.ResponseWriter, partial bool) {
+	w.Header().Set(dataSourceHeader, "local")
+	if partial {
+		w.Header().Set(dataFreshnessHeader, "partial")
+		return
+	}
+	w.Header().Set(dataFreshnessHeader, "local")
+}
+
+func markMixedResponse(w http.ResponseWriter, client staleResponseReporter, partial bool) {
+	w.Header().Set(dataSourceHeader, "mixed")
+	if client != nil && client.LastResponseWasStale() {
+		w.Header().Set(dataFreshnessHeader, "stale")
+	} else if partial {
+		w.Header().Set(dataFreshnessHeader, "partial")
+	} else {
+		w.Header().Set(dataFreshnessHeader, "local")
+	}
+}