feat(kv-cache): allocate mixed MLA groups in lockstep

[FujitsuPolycom]

Summary

• add a tightly gated lockstep allocation mode for mixed DCP-sharded and DCP-replicated MLA cache groups
• share physical block IDs while retaining distinct contiguous cache tensors and group-specific prefix hashes
• preserve existing allocation/packing behavior outside the mixed-MLA predicate
• reject external KV loads for this mode until their ownership semantics are defined

Dependency role

This is the allocator prerequisite for a replicated sparse-indexer K cache beside a DCP-sharded main MLA CKV cache. It contains no model wiring, Sparkinfer transport, prefetch, or sparse-decode policy.

The fork-only base branch contains the current Sparkinfer namespace migration plus GG prerequisite PRs local-inference-lab#128, local-inference-lab#130, local-inference-lab#131, local-inference-lab#132, local-inference-lab#136, local-inference-lab#137, and local-inference-lab#138, which GitHub records as merged. Those prerequisites are intentionally excluded from this feature diff.

Validation

• 7 focused lockstep/prefix/cache tests passed
• 161 adjacent core tests passed; remaining broad-run failures were test-environment device-detection cases
• ai01 CUDA slot-mapping test passed
• Ruff, py_compile, and git diff --check passed

Test host: ai01 is an ASUS Pro WS WRX90E-SAGE SE system with an AMD Ryzen Threadripper PRO 9965WX, 128 GiB RAM, and 4x NVIDIA RTX PRO 6000 Blackwell 96 GB GPUs at 400 W, each on a PCIe 5.0 x16-capable link; IOMMU is off.

Scope

Fork-only draft for integration validation. No upstream PR has been opened.

Design document

GLM-5.2 Sparse CKV under Decode Context Parallelism is the canonical architecture, configuration, validation, and upstream-decomposition reference for this PR stack.

[github-actions]

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