fix: session-export timings tolerate corrupt timestamp cells

`_export_timings` parsed message timestamps with a bare `float()`, so a
pre-existing out-of-range cell (e.g. 8.4e252) reached
`datetime.fromtimestamp` and raised OverflowError, killing the whole
`export_session`/`export_all` call — a regression caught by
tests/hermes_state/test_corrupt_row_robustness.py. Route the cell through
`coerce_epoch`, the reader every other timestamp surface already uses, so a
bad row degrades to a `missing` count (with the usual warning naming the
session) and the export still lands.

hermes_state_portability.py

@@ -81,7 +81,7 @@ def _rich_select(select_cols: str, where: str, tail: str = "", prompt_select: Op
 _PROMPT_RESOLVED_SQL = "COALESCE(sp.prompt, s.system_prompt) AS _system_prompt_resolved"
 
 
-def _export_timings(messages: List[Dict[str, Any]]) -> Dict[str, Any]:
+def _export_timings(messages: List[Dict[str, Any]], session_id: Optional[str] = None) -> Dict[str, Any]:
     """Text-free timing evidence for a session export (port of nearai/ironclaw#7735).
 
     Exports get attached to bug reports; a reader should not have to infer from raw
@@ -89,13 +89,11 @@ def _export_timings(messages: List[Dict[str, Any]]) -> Dict[str, Any]:
     intervals. Hermes persists no model/tool stopwatch samples, so message
     timestamps are the durable floor (``complete`` is therefore always False).
     Ids, roles, counts and durations only — never prompt text, arguments or results.
+    Corrupt timestamp cells go through ``coerce_epoch`` like every other reader: they
+    count as ``missing`` and never abort the export.
     """
-    timestamped = []
-    for msg in messages:
-        try:
-            timestamped.append((msg, float(msg.get("timestamp"))))
-        except (TypeError, ValueError):
-            continue
+    timestamped = [(msg, ts) for msg in messages
+                   if (ts := coerce_epoch(msg.get("timestamp"), session_id=session_id)) is not None]
     role_counts = Counter(str(msg.get("role") or "unknown") for msg in messages)
     tool_calls_emitted = sum(
         len(tc) if isinstance(tc, list) else 1 for tc in (msg.get("tool_calls") for msg in messages) if tc)
@@ -280,7 +278,7 @@ class SessionPortabilityMixin:
 
     def _with_messages(self, session: Dict[str, Any]) -> Dict[str, Any]:
         messages = self.get_messages(session["id"])
-        return {**session, "messages": messages, "timings": _export_timings(messages)}
+        return {**session, "messages": messages, "timings": _export_timings(messages, session["id"])}
 
     def export_session(self, session_id: str) -> Optional[Dict[str, Any]]:
         """Export a single session with all its messages as a dict."""
@@ -320,7 +318,7 @@ class SessionPortabilityMixin:
                     self._row_to_message_dict(row, warn_context="get_messages", summary_flag=True)
                 )
         return [{**session, "messages": messages_by_session[session["id"]],
-                 "timings": _export_timings(messages_by_session[session["id"]])} for session in sessions]
+                 "timings": _export_timings(messages_by_session[session["id"]], session["id"])} for session in sessions]
 
     def adopt_session_lineage_from(self, donor_db: Any, session_id: str, *, retire_donor: bool = True) -> Dict[str, Any]:
         """Adopt *session_id*'s full compression lineage from *donor_db* (stranded-bot-session