[Feature] Add serving contract support for lib-serving decoupling

[bumble0918]

Summary

Add a pypto-lib owned serving contract layer so pypto-serving can consume model kernel ABI details through structured contracts instead of duplicating lib internals.

Motivation / Use Case

The serving-side architecture work is tracked in hw-native-sys/pypto-serving#65. To complete the decoupling, pypto-lib should become the source of truth for model-specific serving ABI details, including kernel loading, host-side JIT wrapper signatures, compile/runtime argument builders, weight layout preparation, and compatibility validation.

This keeps pypto-serving focused on runtime orchestration while allowing Qwen3-14B, DeepSeek-V4 Flash/Pro, and future models to expose their serving requirements through lib-owned contracts.

Proposed API / Behavior

Introduce pypto-lib support for:

• a serving contract registry that selects a ModelServingContract from explicit family/variant or structured model config metadata;
• ModelServingContract metadata for model identity, capabilities, limits, KV/cache layout, execution graph, kernel specs, and weight metadata;
• lib-owned kernel loaders and validators;
• lib-owned host-side JIT wrappers, compile argument builders, runtime argument builders, and weight layout preparation hooks;
• structured compatibility errors including schema version, ABI fingerprint, incompatible field, and suggested action.

Detailed design reference: hw-native-sys/pypto-serving#65

Alternatives Considered

No response

Additional Context

No response