feat: Qwen3.5 V2 VM bytecode runtime support

[nuri-yoo]

Summary

Adds two pieces required to load and execute V2 VM bytecode artifacts that mlc-llm produces for Qwen3.5 (and other Gated DeltaNet hybrid) models on stock relax runtimes.

Changes

src/runtime/vm/attn_backend.cc — Accept both \"tirx\" (current) and \"tir\" (older artifact name) at every match site inside ConvertPagedPrefillFunc / ConvertRaggedPrefillFunc / ConvertPagedDecodeFunc / Convert*-TreeMaskFunc. Strictly widens the accepted set; no rejection path changes.

src/runtime/device_c_api.cc (new, 108 LoC) — Flat C-ABI wrapper around tvm::runtime::DeviceAPI::Get(...). Re-exports TVMDeviceAPIGet / TVMDeviceAPICopyDataFromTo / TVMDeviceAPIStreamSync etc. so Rust crates that link against the C symbols can use this build without re-deriving the wrapper. Strict re-export, no logic.

Verification

End-to-end on macOS arm64 + Metal:

• Qwen3-0.6B / Qwen3-8B (V1 KvCache)
• Qwen3.5-0.8B / 2B / 4B / 9B (V2 hybrid Gated DeltaNet)
• BAAI/bge-m3 (V1 embedding)

All load and run with semantically correct outputs.

Test plan

• Build relax with the new branch and verify existing V1 artifacts still load
• Build the matching mlc-llm against this relax and verify V2-bytecode rt.dylib loads
• Run an external Rust consumer (e.g. ailoy) that links against the C-ABI shim