fix(issue-5): address phase 2a round 2 findings (F-C, F-D, R1, R4)

Both reviewers sustained the round 1 fixes and found four more. Each was
reproduced here before being accepted; none were rebutted.

F-C (claude, blocking) -- my own round 1 fix caused this. Making refused writes
exit non-zero also classified an idempotent ACK as a refusal, and the wrapper
finalises an approved job with `status --set completed` on a record that is
already completed. It runs under `set -e`, so the *success* path aborted with
exit 4 before its validation hook and its final stdout line. Reproduced at the
shell. An idempotent result is now distinguishable from a refusal and exits 0; a
genuine conflict still exits 4.

F-D (claude) -- routing the operator write around update_status dropped the
status vocabulary check. Under the rollback flag an invalid status was written
to the record with exit 0, where before it was refused with exit 1 and the record
left alone. The check now runs first, in both modes.

R1 (codex, blocking) -- the previous fix compared identifiers only when they
happened to be present, so evidence carrying none at all was still approved on
the dispatch record's own flags. That is the self-assertion the check exists to
replace. Every identifier must now be supplied and agree, and a report manifest
must associate with the attempt being judged. Identifiers are read from a
collector envelope beside the payload rather than from inside it: a native Herdr
payload may not carry MAM fields, so one appearing there is either fabricated or
a contract violation, and either way it is not evidence.

R4 (codex) -- a sequence number orders nothing unless the compare and the write
are atomic. Two flushes could both read the old value, the newer land first, and
the slower overwrite it. Reproduced deterministically. The mirror now serialises
read-compare-write under a per-job audit lock, taken outside the registry
transaction so a slow mirror still cannot lengthen it, and each write uses its
own temp file instead of a shared name.

All eight new regressions fail against the pre-fix code.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UrDNJkHmm18uu7n7JwV5Qd

.agents/skills/lib_py/delegation/engine.py

@@ -81,10 +81,16 @@ def cas_enabled() -> bool:
 
 
 class TransitionResult(object):
-    __slots__ = ("committed", "from_status", "to_status", "reason", "record", "notifications")
+    __slots__ = ("committed", "from_status", "to_status", "reason", "record",
+                 "notifications", "idempotent")
 
-    def __init__(self, committed, from_status, to_status, reason, record, notifications):
+    def __init__(self, committed, from_status, to_status, reason, record, notifications,
+                 idempotent=False):
         self.committed = committed
+        # Not every uncommitted transition is a refusal. Re-receiving a terminal
+        # that already holds is a no-op ACK in the contract table, and a caller
+        # that treats it as failure turns a success path into an error.
+        self.idempotent = idempotent
         self.from_status = from_status
         self.to_status = to_status
         self.reason = reason
@@ -97,13 +103,14 @@ class TransitionResult(object):
 
 
 class _Decision(object):
-    __slots__ = ("commit", "to_status", "reason", "conflict")
+    __slots__ = ("commit", "to_status", "reason", "conflict", "idempotent")
 
-    def __init__(self, commit, to_status, reason, conflict=False):
+    def __init__(self, commit, to_status, reason, conflict=False, idempotent=False):
         self.commit = commit
         self.to_status = to_status
         self.reason = reason
         self.conflict = conflict
+        self.idempotent = idempotent
 
 
 def _decide(record: Dict[str, Any], request: str, expected_status, expected_attempt,
@@ -156,7 +163,8 @@ def _decide(record: Dict[str, Any], request: str, expected_status, expected_atte
     if current in TERMINAL_STATUSES:
         # Idempotent re-receipt of the same terminal is a no-op ack.
         if requested == current:
-            return _Decision(False, current, "idempotent: already {}".format(current))
+            return _Decision(False, current, "idempotent: already {}".format(current),
+                             idempotent=True)
 
         operator = source in OPERATOR_SOURCES
         if operator and request == "redispatch":
@@ -249,7 +257,8 @@ def transition(job_id, request="merge", *, registry_dir=None, expected_status=No
         logger.warning("refused %s for job %s: %s", request, job_id, decision.reason)
 
     return TransitionResult(decision.commit, from_status, to_status,
-                            decision.reason, record, notifications)
+                            decision.reason, record, notifications,
+                            idempotent=decision.idempotent)
 
 
 def _flush(notifications, registry_dir) -> None: