[NPU] keep Qwen3.6 hybrid SSM state stable across radix modes

[McZyWu]

Summary: restore the stable torch_npu NPU MRoPE path, separate the kernel intermediate-state chunk size from the radix checkpoint grid, and split an unaligned Mamba prefill tail even when ChunkCache disables radix caching. This prevents padding rows and incorrect h-state indices from advancing the Qwen3.6 GDN recurrent state. Companion native AscendC causal_conv1d fix: sgl-project/sgl-kernel-npu#742. Validation: on machine 152 with NEXTN enabled, radix cold/hot and recold passed 5 of 5, and no-radix passed 2 of 2 with identical stopped output. Unit coverage includes state-grid indexing and no-radix tail splitting. Machine 3 validation is pending because its SSH endpoint is currently unreachable.

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CI States

Latest PR Test (Base): ❌ Run #32724871949
Latest PR Test (Extra): ❌ Run #32724871530
Latest PR Test (AMD ROCm 7.2): ❌ Run #32724871929

[McZyWu]

Machine-3 E2E validation completed on container sglwmc-821-m with Qwen3.6-35B-A3B, TP=2, Ascend attention, BF16 Mamba state, NEXTN enabled (3 steps), and the native AscendC conv1d fix from sgl-kernel-npu#742.

The production test was copied byte-for-byte except port 8998 -> 8898 so it could not hit the unrelated sglwmc-823 service.

Results:

• Radix enabled: cold + 3 hot-prefix runs + cold-after-restart = 5/5 pass
• Radix disabled: 3/3 pass
• All 8 outputs: finish_reason=stop, 1319 SSE events, 11503 content chars, zero JSON parse errors, zero replacement characters
• All 8 outputs were byte-identical after SSE reconstruction: SHA256 535bb041ed219fc53efde40bb1d83755437d48d521b6b0c3680c1f12a7529098
• Radix hot runs reported 29056 cached tokens
• Cold/no-radix physical chunks were 16384 + 12672 + 128, exercising the aligned Mamba tail path
• Longest repeated suffix was 797 hyphens; this is explicitly accepted by the reporter and generation stopped normally

Runtime verification:

• Tested functional SGLang HEAD: 3815a17
• Current 488d387 only removes one isort-requested blank line
• Loaded native library SHA256: 5fc99caf79b816955bf10026a65c59c403a34faedd5425524ecf6a125e08a8a8
• /proc maps for the server and both TP workers all point to the fixed libsgl_kernel_npu.so

The radix service remains healthy on container sglwmc-821-m, port 8898, devices 4/5.