v0.297.0: burn-down round 2 — 24 small rows (R-591 R-568 R-567 R-363 R-547 R-10 R-552 R-251 R-104 R-619 R-362 R-675 R-256 R-257 R-240 R-365 R-425 R-565 R-564 R-603 R-454 R-208 R-457-swept) + the banner countdown and deepCopyStack twins; MinAgent 0.131.0

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/backup/backup.go

@@ -155,6 +155,9 @@ type Manager struct {
 	// offboxFreeFn (3a) — the free-space probe for the restore headroom gate, overridable in tests (the
 	// Windows `go test` host has no `df`). Nil → the real diskFreeBytes (df --output=avail).
 	offboxFreeFn func(path string) int64
+	// driveMountedFn (R-362) answers "is this registered drive still mounted?" when a restore cannot
+	// create its directory. nil → system.IsMountPoint. Tests inject it; production never sets it.
+	driveMountedFn func(path string) bool
 
 	// offboxLatestSnapFn (R-357) overrides the restic snapshot lookup, and it exists for one reason:
 	// without it, ReconstituteFromOffsite's new headroom gate cannot be tested at the level that