PINK: fix naive datetime → INVALID_INTENT_PARSE at column 41

Root cause: intent.timestamp was a naive datetime (no tzinfo). isoformat()
produces '2026-06-04T14:26:55.098914' (26 chars). The JSON prefix
'{"timestamp":"' is 14 chars → closing quote lands at column 41. Rust's
chrono::DateTime<Utc> serde rejects naive timestamps and serde_json reports
the error as 'premature end of input at line 1 column 41'.

Fix: _utc_isoformat() attaches UTC tzinfo before isoformat(), producing
'2026-06-04T14:26:55.098914+00:00' which chrono accepts.

Previous null-byte fix (_to_rust_bytes) and dangling-pointer fix (local vars)
remain correct and address real separate failure modes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prod/clean_arch/dita_v2/rust_backend.py

@@ -437,9 +437,25 @@ def _first_invalid_intent_field(intent: KernelIntent) -> Optional[tuple[str, flo
     return None
 
 
+def _utc_isoformat(ts: Any) -> str:
+    """Return an RFC3339 timestamp string with UTC offset (+00:00).
+
+    Rust's chrono::DateTime<Utc> serde deserializer requires a timezone-aware
+    string. A naive datetime (no tzinfo) produces e.g. '2026-06-04T14:26:55.098914'
+    (26 chars) which chrono rejects with 'premature end of input at column 41'
+    (14-char JSON key prefix + 26-char value + closing quote = column 41).
+    """
+    from datetime import timezone as _tz
+    if hasattr(ts, "tzinfo"):
+        if ts.tzinfo is None:
+            ts = ts.replace(tzinfo=_tz.utc)
+        return ts.isoformat()
+    return str(ts)
+
+
 def _intent_to_payload(intent: KernelIntent) -> Dict[str, Any]:
     return {
-        "timestamp": intent.timestamp.isoformat() if hasattr(intent.timestamp, "isoformat") else str(intent.timestamp),
+        "timestamp": _utc_isoformat(intent.timestamp),
         "intent_id": intent.intent_id,
         "trade_id": intent.trade_id,
         "slot_id": intent.slot_id,