feat(disagg): unify per-layer transfer over chunked prefill (lever A)

Per code review (HiCache load is layer-by-layer & correctly ordered into the compute
stream before the backup hook; current-reuse is a within-forward read that doesn't
rewrite pool pages): unify registration to the EXACT range this forward's
send_kv_chunk transmits — req_to_token[start_send_idx:end_idx], page-floored for a
non-last chunk. Non-chunked = one full range; chunked = one range per chunk. Drop
the is_chunked/start_send_idx skip.

To avoid the review's collision risk (chunk N still finishing when chunk N+1
registers), the manager keys contexts per (room, start_send_idx): _active[room] is a
FIFO deque of (chunk_key, ctx); register dedups the same chunk but appends a new one;
finish(room) pops the FRONT (chunks finish in send order — no chunk key needed in the
transfer_worker); drop drains all the room's chunks; on_layer_end enqueues for all
active chunk contexts (per-ctx note_enqueued dedup keeps each chunk's own events).

28 unit tests pass incl. chunked FIFO + per-chunk dedup.

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

test/registered/unit/disaggregation/test_cp_per_layer_transfer.py

@@ -250,6 +250,25 @@ class TestPerLayerTransferManager(unittest.TestCase):
         # boundaries at L=3 (group [0,3]) and L=7 (group [4,7]) -> 2 groups, not 8
         self.assertEqual([(s, e) for _, s, e, _ in items], [(0, 3), (4, 7)])
 
+    def test_chunked_fifo_separate_contexts_and_dedup(self):
+        m = _manager(group_size=1)
+        c1, c2 = _MockCtx(), _MockCtx()
+        m.register("r1", c1, chunk_key=0)  # chunk 1
+        m.register("r1", c1, chunk_key=0)  # SAME chunk re-register -> deduped
+        m.register("r1", c2, chunk_key=64)  # chunk 2 (different start_send_idx)
+        self.assertTrue(m.has_chunk("r1", 0))
+        self.assertTrue(m.has_chunk("r1", 64))
+        self.assertFalse(m.has_chunk("r1", 128))
+        m.on_layer_end(5)  # both chunk contexts active -> both enqueued
+        self.assertEqual(len(_drain(m._q)), 2)
+        # finish pops FIFO: chunk 1 (c1) first, then chunk 2 (c2)
+        self.assertEqual(m.finish("r1"), 0)
+        self.assertTrue(c1.finished)
+        self.assertFalse(c2.finished)
+        self.assertEqual(m.finish("r1"), 0)
+        self.assertTrue(c2.finished)
+        self.assertFalse(m.has_room("r1"))
+
     def test_worker_step_calls_submit_group(self):
         m = _manager()
         c = _MockCtx()