Make the duplicate-reservation ack gate O(1) for fresh node ids

node_has_undrained_write_ack scanned ack_write_queue on every
reservation, including the per-request-per-chunk prepare path whose
node ids are freshly minted and can never be queued. Track the max
node id ever appended to the queue: any queued id is <= max by
construction (no monotonicity assumption needed for correctness), so
fresh ids exit on one integer compare and the scan remains only for
re-reservations of old ids (the rare write_backup fallback).

All three ack_write_queue append sites now go through a single
_append_write_ack funnel that maintains the max — the zero-owned-rank
and non-CP write acks previously would have bypassed it, allowing
false negatives on ranks owning no pages of a node.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/registered/unit/mem_cache/test_cp_hicache_metadata.py

@@ -2176,6 +2176,17 @@ class TestHiRadixCacheCPBackup(CustomTestCase):
         self.assertEqual(cache.ongoing_write_through, {53: attached_node})
         self.assertEqual(dec_locked, [])
 
+    @staticmethod
+    def _make_exploding_queue():
+        class _ExplodingQueue(list):
+            def __iter__(self):
+                raise AssertionError("fresh node ids must not scan the ack queue")
+
+            def __bool__(self):
+                return True
+
+        return _ExplodingQueue()
+
     def test_reserve_write_cp_refuses_node_with_undrained_ack(self):
         """Producer-side invariant: at most one in-flight ack per node_id.
         A reservation for a node whose previous final ack is still queued is
@@ -2190,6 +2201,7 @@ class TestHiRadixCacheCPBackup(CustomTestCase):
             ack_write_queue=[
                 HiCacheAck(MagicMock(), MagicMock(), [4614]),
             ],
+            _max_write_ack_node_id=4614,
         )
         controller.node_has_undrained_write_ack = types.MethodType(
             HiCacheController.node_has_undrained_write_ack, controller
@@ -2201,9 +2213,15 @@ class TestHiRadixCacheCPBackup(CustomTestCase):
         )
         self.assertIsInstance(result, HiCacheWriteFailure)
         self.assertEqual(result.required_host_slots, 0)
-        # Different node id is not gated by the queue scan.
+        # An old (<= max) but absent node id is resolved by the queue scan.
         self.assertFalse(
-            HiCacheController.node_has_undrained_write_ack(controller, 4615)
+            HiCacheController.node_has_undrained_write_ack(controller, 4613)
+        )
+        # Fresh ids (> max ever acked, i.e. every prepare-path reservation)
+        # take the O(1) fast path: no queue scan at all.
+        controller.ack_write_queue = self._make_exploding_queue()
+        self.assertFalse(
+            HiCacheController.node_has_undrained_write_ack(controller, 9999)
         )
 
     def test_rollback_pending_backup_scrubs_acks_and_cancels_state(self):