R-86: restore-test follows the backup, not the clock (v0.121.0)

The ticker survives as the EVALUATION interval only. A tier is DUE when its
newest archive that has settled for `settle` (default 24h) has not been proven:
daily tier -> proved daily on yesterday's archive, weekly tier -> weekly on its
own, newborn -> UNKNOWN.

The trap avoided: the literal reading ("newest archive is >= 24h old") is NEVER
true on a daily tier, so it silently switches restore-testing off where it
matters most. Red-proved at 0 runs over 5 simulated days.

- state records WHICH archive was proven; legacy files keep their time and yield
  no proven archive (each tier due once after the upgrade, deliberately)
- two knobs replace one: restore_test_eval_interval_seconds (6h, measured) and
  restore_test_settle_seconds (24h). The old cadence key keeps its DISABLE
  meaning verbatim and now seeds the settle lag, with a start-up WARN.
- due-check runs BEFORE the heavy-op gate (a frequent poll must not make a
  starting backup record a failure, F-A1)
- candidate picker skips implausible archives (a phantom would be due forever)
- new read-only --selftest=restore-test-due prints the verdict + its cost

internal/backup/runner.go

@@ -260,21 +260,59 @@ func (r *BackupRunner) PickRestoreCandidate(ctx context.Context) (string, error)
 // restore is not an error** — a brand-new offsite tier legitimately has nothing yet, and turning
 // that into a failure would make every fresh box look broken for its first week.
 func (r *BackupRunner) PickRestoreCandidateOn(ctx context.Context, target string) (string, error) {
+	archive, _, err := r.PickSettledRestoreCandidateOn(ctx, target, time.Time{})
+	return archive, err
+}
+
+// PickSettledRestoreCandidateOn is the R-86 due-check's picker: the newest archive on target that
+// landed AT OR BEFORE notAfter (the settle cutoff), with the time it landed. A zero notAfter means
+// "no cutoff" — that is the pre-R-86 behaviour, which is why PickRestoreCandidateOn is now a
+// one-line call into this and its contract is untouched (one scan, one owner).
+//
+// WHY A CUTOFF AT ALL. An archive that landed minutes ago may still be settling — R-71a's
+// settle-gate exists because the offsite tier's day-0 consume raced its own floor update — and
+// restore-testing the archive a backup is still writing proves nothing about the backup that
+// finished. The due-check therefore asks about the newest SETTLED archive, and §8.1's rule is built
+// on that: the tier is due when a settled archive exists that has not been proven.
+//
+// The plausibility floor is applied here and not in the old path on purpose. Under R-86 the picked
+// archive becomes the tier's due-ness: an incomplete 1-byte phantom (F-CRIT-2's artefact — server
+// prune does NOT collect it) would be selected forever, fail its restore forever, never earn proof,
+// and so make the tier due at EVERY evaluation. Skipping it is what keeps the retry rate bounded by
+// the archive generation rather than by the evaluation interval.
+//
+// Contract preserved: ("", zero, nil) when the storage holds no eligible archive. **A tier with
+// nothing to restore is not an error** — a brand-new offsite tier legitimately has nothing yet, and
+// turning that into a failure would make every fresh box look broken for its first week.
+func (r *BackupRunner) PickSettledRestoreCandidateOn(ctx context.Context, target string, notAfter time.Time) (string, time.Time, error) {
 	if target == "" {
-		return "", nil
+		return "", time.Time{}, nil
 	}
 	contents, err := r.api.StorageContent(ctx, target)
 	if err != nil {
-		return "", err
+		return "", time.Time{}, err
 	}
 	var best string
 	var bestCTime int64 = -1
 	for _, e := range contents {
-		if e.Content == "backup" && e.CTime > bestCTime {
+		if e.Content != "backup" {
+			continue
+		}
+		if !notAfter.IsZero() && e.CTime > notAfter.Unix() {
+			continue // not settled yet — a newer archive is not a reason to re-prove an older one
+		}
+		if ok, why := archivePlausiblyComplete(e); !ok {
+			r.warnRejectedArchiveOnce(e, why)
+			continue
+		}
+		if e.CTime > bestCTime {
 			bestCTime, best = e.CTime, e.VolID
 		}
 	}
-	return best, nil
+	if best == "" {
+		return "", time.Time{}, nil
+	}
+	return best, time.Unix(bestCTime, 0).UTC(), nil
 }
 
 // latestArchive finds the newest backup archive volid + size for vmid on the target.