[II] Replicate Kimi DSpark cache under target DCP

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Behavior

Kimi-K3 DSpark may run with a B12X MLA target that uses decode context parallelism. The external draft keeps a complete DCP1 KV cache on every target rank unless VLLM_DCP_SHARD_DRAFT=1 explicitly requests draft sharding.

The speculative configuration accepts target DCP only when the external Kimi draft selects B12X_MLA; other attention backends retain the DCP1 requirement.

Technical reason

Kimi-K3 target KV is partitioned by DCP, while an external DSpark model executes independently on every target rank. Marking the non-causal draft MLA cache as dcp_replicated gives each rank complete draft context without changing the target cache layout.

Compatibility

• Causal Kimi-K3 target layers are never marked as replicated draft cache.
• VLLM_DCP_SHARD_DRAFT=1 preserves explicit draft-cache sharding.
• DCP1 behavior is unchanged.
• Runtime activation requires the backend-owned Kimi B12X DCP implementation in vLLM [II] Select backend-owned Kimi dense MLA DCP #360.
• This pull request is stacked on vLLM [II] Isolate external draft attention geometry #363.

Validation

• Ruff and Python compilation pass for all changed files.
• Six Kimi DSpark configuration and cache-replication tests pass in the source-locked CUDA 13.3 / PyTorch 2.13 runtime image.

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[voipmonitor]

The behavior implemented by this pull request is preserved in vLLM #389 as commit 770b244d7f1f. The stable patch ID differs because the DSpark DCP tests share imports and setup with the consolidated draft online-quantization coverage. The consolidated range-diff retains the pull request contract. Review and merge #389; this pull request is closed to avoid duplicate review.