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>

python/sglang/srt/disaggregation/mooncake/conn.py

@@ -395,7 +395,7 @@ class MooncakeKVManager(CommonKVManager):
             self.engine, mooncake_session_id, get_blocks, num_layers=n_layers
         )
 
-    def register_per_layer_transfer(self, room, page_indices) -> bool:
+    def register_per_layer_transfer(self, room, page_indices, chunk_key=0) -> bool:
         """Lever A: before the forward, build + register a per-layer transfer context
         for `room` so the per-layer notifier overlaps its main-KV transfer with the
         forward. Reuses send()'s CP filter exactly (no re-derivation). Scoped to
@@ -428,10 +428,11 @@ class MooncakeKVManager(CommonKVManager):
                 info.mooncake_session_id, owned_pages, dst_indices
             )
             if ctx is not None:
-                mgr.register(room, ctx)
+                mgr.register(room, ctx, chunk_key=chunk_key)
                 logger.info(
-                    "[CP_PER_LAYER_TRANSFER] registered room=%s owned_pages=%d",
+                    "[CP_PER_LAYER_TRANSFER] registered room=%s chunk=%s owned_pages=%d",
                     room,
+                    chunk_key,
                     len(owned_pages),
                 )
                 return True