[Juntian777]

Follow-up cleanup on top of perf/dsv41-cut-layer-trim (vllm-project#59894).

Changes

• The cut layer's full-batch KV write now runs inside DecoderReplayLayers.__call__ (write_batch_kv), so the KV write and the PIECEWISE graph break share one decision, uses_replay_batch().
• forward_kv inserts the SWA KV with the KV-only fused_deepseek_v4_kv_rope_insert (as DSpark's _insert_context_kv does) instead of a placeholder query. It runs the same KV branch of the same kernel; the cache bytes match for packed (with and without MXFP8), bf16 and fp8 caches.
• Assert the last KV source is not ratio-2: rerunning its compressor on the replay rows would split groups, and the replay batch builds no state-cache metadata for it. DSv4.1-Flash's cut layer (20) is ratio-1.
• Merge the two replay attention-group caches, rename replays / graph_replays to uses_replay_batch / graph_uses_replay_batch, and shorten comments.
• Tests assert that the KV write follows the replay decision (including a PIECEWISE capture with no replay batch set) and that the runtime replay batch and the capture agree at the threshold boundary.

Tests

• pytest tests/models/test_deepseek_v41_{decoder_replay_layers,replay_batch,replay_start}.py: 15 passed. Gating on replay_batch alone, or using > instead of >= at runtime, each fails one test.
• DSv4.1-Flash on 4x GB200, DEP4 + DSpark (5 speculative tokens, adaptive verification), Mega Attention + Mega MoE, CUDA graphs up to 8190 tokens (decoder replay graphs captured):

Eval	Score	DSpark acceptance length (incl. bonus)
GSM8K, lm_eval 5-shot strict-match, greedy, 1024 tokens, concurrency 32	92.80%	3.426
GPQA Diamond, 0-shot chat, thinking, T=1.0, top_p=0.95, 262144-token cap	91.41%	3.169

An earlier DEP4 + DSpark GSM8K run with the same server flags scored 92.72% / 92.80% with acceptance 3.41.

AI assistance (Claude Code) was used; I reviewed every line and ran the tests above.

🤖 Generated with Claude Code

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[Juntian777]

@gcanlin I made some extra cleanups on top of your PR. Could you please take a look? Thanks!

[gcanlin]

Thanks!