Share unavailable() so the Windows sensor logs unavailable transitions

internal/evidence/unavailable.go

@@ -0,0 +1,10 @@
+package evidence
+
+import "log"
+
+// unavailable builds an Unavailable snapshot and logs the reason. It is shared
+// by every platform sensor so an unobservable window is recorded consistently.
+func unavailable(reason string) WindowSnapshot {
+	log.Printf("evidence: %s", reason)
+	return WindowSnapshot{Health: EvidenceHealth{Available: false, Reason: reason}}
+}