fix(safety): il boot-check dell'audit tollera il lag benigno dell'anchor

L'anchor dell'audit (system_state.last_audit_hash) e' persistito best-effort:
sotto contesa di lock SQLite il mirror puo' restare indietro rispetto al log,
che invece cresce in avanti integro. Il check di boot armava il kill-switch
su qualsiasi disuguaglianza anchor!=coda, scambiando questo lag per
manomissione.

- audit_log: nuova tail_continues_from() — True solo se l'anchor e' un
  antenato valido della coda (presente in catena + chain forward integra a EOF)
- orchestrator._verify_audit_anchor: se lag benigno -> re-sync anchor + warning;
  solo anchor assente o chain rotta (troncamento/tamper) -> critical -> arma

Verificato live: anchor in ritardo -> re-sync senza armare; anchor bogus -> arma.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/cerbero_bite/safety/audit_log.py

@@ -28,6 +28,7 @@ __all__ = [
     "AuditChainError",
     "AuditEntry",
     "AuditLog",
+    "tail_continues_from",
     "verify_chain",
 ]
 
@@ -157,6 +158,55 @@ def verify_chain(path: str | Path) -> int:
     return count
 
 
+def tail_continues_from(path: str | Path, anchor_hash: str) -> bool:
+    """Return ``True`` when *anchor_hash* is a genuine ancestor of the tail.
+
+    That is: *anchor_hash* is the ``hash`` of some line in the log AND every
+    line after it forms a valid forward chain to EOF. This distinguishes a
+    **benign anchor lag** — the best-effort anchor persisted in
+    :func:`cerbero_bite.runtime.dependencies.build_runtime` fell behind the
+    file under SQLite write contention, yet the log grew forward
+    legitimately — from real **truncation/tampering** (anchor absent, or the
+    post-anchor chain broken).
+
+    Returns ``False`` if the file is missing/empty, the anchor is not found,
+    or the chain from the anchor to the tail does not verify. The single-
+    writer invariant of :class:`AuditLog` still holds; this only makes the
+    boot-time anchor check tolerant of the durability gap it documents.
+    """
+    p = Path(path)
+    if not p.exists() or p.stat().st_size == 0:
+        return False
+    seen = False
+    expected_prev = ""
+    with p.open("r", encoding="utf-8") as fh:
+        for line in fh:
+            if not line.strip():
+                continue
+            try:
+                entry = _parse_line(line)
+            except AuditChainError:
+                if seen:
+                    return False
+                continue
+            if seen:
+                if entry.prev_hash != expected_prev:
+                    return False
+                recomputed = _compute_hash(
+                    entry.timestamp.isoformat(),
+                    entry.event,
+                    _canonical_payload(entry.payload),
+                    entry.prev_hash,
+                )
+                if recomputed != entry.hash:
+                    return False
+                expected_prev = entry.hash
+            elif entry.hash == anchor_hash:
+                seen = True
+                expected_prev = entry.hash
+    return seen
+
+
 def iter_entries(path: str | Path) -> Iterator[AuditEntry]:
     """Yield each :class:`AuditEntry` from *path* without verifying."""
     p = Path(path)