[WIP] feat(config): runtime config decoupling(design for reference)

[rjzhb]

Summary

Legacy config flows pass HuggingFace PretrainedConfig objects straight into ModelConfig and model code. When transformers changes field names, nesting, or defaults, bugs show up deep in the runtime and fixes spread across many files.

This PR adds an engine-owned EngineModelSpec IR between HF parsing and the runtime. HF config is translated once per model in an adapter (hf_config → EngineModelSpec). Engine code should depend on the spec, not the HF schema. After a transformers update, we usually only fix parsing or the adapter—not model/loader code.

Pilots: minimax_m2, qwen3_5 / qwen3_5_moe
Flag: TOKENSPEED_USE_ENGINE_SPEC=1 (default off; legacy path unchanged)

Shared spec design

Both pilots use the same EngineModelSpec entry type—not separate per-model config systems.

Shared shell — every model produces the same top-level shape:

EngineModelSpec { schema_version, model_type, architecture, dtype, quantization, body }

Shared components — architecture-specific bodies are built from reusable blocks:

• GQAAttentionSpec — attention heads / KV / RoPE dim
• MoEMLPSpec — expert count, top-k, routing
• RMSNormSpec, RopePositionSpec — norm and position encoding

MiniMax-M2 and Qwen3.5 both use these; only the adapter mapping from HF fields differs.

Typed body union — model-specific details live in body, not the shell:

ModelBody = MinimaxM2ModelSpec | Qwen35ModelSpec

• MiniMax-M2: shared components + MTP fields (num_mtp_modules, …)
• Qwen3.5: shared components + hybrid extras (GatedDeltaNetSpec, full_attention_interval, dense/MoE sizes, …)

Single dispatch — build_engine_spec() routes by model_type to adapters/minimax_m2 or adapters/qwen3_5; both return EngineModelSpec. ModelConfig then branches on spec.body.type only for the RuntimeView bridge.

When need New models: add a body variant + adapter, reuse existing components where possible—no new flat config type per model.

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