v0.133.0: a restore-test can never fill a box's disk; leftovers retried on a timer (R-672, R-673)

Space preflight before anything is created (uncompressed size from the vzdump log / PBS
snapshot, x1.2 + 5 GiB, thin metadata, off the tested guest's pool when another storage
is eligible, unknown refuses, reported as a non-pass result). Failed scratch teardown and
the stale-lock sweep retried every 10 min (the sweep under the heavy-op gate). A thin
pool crossing 90% requests an immediate host report. Six red-proofs.

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

internal/config/config.go

@@ -345,6 +345,12 @@ type BackupConfig struct {
 	// between an archive settling and its proof, and the retry rate of a tier whose restore-test
 	// keeps failing. See defaultRestoreTestEvalInterval for the measurement it was chosen from.
 	RestoreTestEvalIntervalSeconds int `json:"restore_test_eval_interval_seconds"`
+	// RestoreTestSpaceFactor / RestoreTestSpaceReserveGiB are the restore-test's space margin (R-672,
+	// v0.133.0): a test starts only when the target storage has free ≥ restored × factor + reserve,
+	// `restored` being the UNCOMPRESSED size. 0/unset → 1.2 and 5 GiB. A test config may raise them to
+	// watch the refusal (the brief's live case a).
+	RestoreTestSpaceFactor     float64 `json:"restore_test_space_factor,omitempty"`
+	RestoreTestSpaceReserveGiB float64 `json:"restore_test_space_reserve_gib,omitempty"`
 	// RestoreTestSettleSeconds is how long an archive must have sat on its tier before it is a
 	// restore-test candidate (R-86); 0 → default (24h), negative → 0 (no settle requirement).
 	// Restore-testing an archive a backup is still writing proves nothing about the backup that
@@ -615,6 +621,20 @@ func (b BackupConfig) RestoreTestEvalInterval() time.Duration {
 	}
 }
 
+// RestoreTestSpace returns the restore-test's space margin (R-672): factor (≥ 1) and reserve bytes.
+// Unset or out-of-range → 1.2 and 5 GiB.
+func (b BackupConfig) RestoreTestSpace() (factor float64, reserveBytes int64) {
+	factor = b.RestoreTestSpaceFactor
+	if factor < 1 {
+		factor = 1.2
+	}
+	reserveBytes = int64(b.RestoreTestSpaceReserveGiB * float64(1<<30))
+	if reserveBytes <= 0 {
+		reserveBytes = 5 << 30
+	}
+	return factor, reserveBytes
+}
+
 // RestoreTestSettle returns how long an archive must have sat before it is a restore-test
 // candidate (R-86): a positive value as-is, negative → 0 (no settle requirement), 0 → the default.
 //