Add MooncakeKVManager.build_per_layer_context: assembles a PerLayerTransferContext
from the SAME CP-filtered (prefill_kv_indices, dst_kv_indices) the post-forward
transfer uses — so the bytes moved are byte-identical to the monolithic path, and
the CP owner mapping is NOT re-derived (eliminating the #1 correctness risk). It
mirrors the MLA branch of _send_kvcache_generic exactly (get_mla_kv_ptrs_with_pp +
group_concurrent_contiguous + build_layer_blocks, verified set_transfer_blocks-
identical). Returns None for MHA / unregistered decode / empty owned set.
Unit-tested (4 cases): per-layer address correctness + the None guards. Remaining
A3-step3: call this in the send/scheduler flow (register before forward, finish
after, skip the main-KV monolithic send), then output-equality + TTFT verification.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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