Wire PerLayerTransferManager into the prefill bootstrap queue: when
SGLANG_CP_SHARED_KV_PER_LAYER_TRANSFER is on, create the manager (real
torch.cuda.Event / current_stream) and register its on_layer_end on the KV pool's
layer_backup_notifiers, exposing it as kv_manager.per_layer_transfer_manager. The
forward's per-layer end hook now reaches the manager. Additive and a no-op until a
request registers a transfer context (A3-step2): on_layer_end returns early when no
contexts are active (verified). Remaining A3 (get_blocks + setup-before-run_batch +
finish reconciled with send_kv_chunk) builds on this.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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