refactor(protocol): enforce abstract contracts and static checks

[QuantumGhost]

AI Disclosure: This PR was generated with Codex and GPT-5.5.

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Related Issue

Closes #136

Summary

• Added @abstractmethod to protocol members that previously lacked it across src/graphon protocol definitions.
• Added static implementation assertions (under if TYPE_CHECKING) for in-repo protocol implementations, following the _assert_readonly_variable_pool pattern.
• Added tests/test_protocol_abstract_contracts.py to enforce protocol abstraction rules and guard against accidental implementation via partial or indirect subclassing.
• Updated type-only imports so imports used only by static assertions are scoped inside the corresponding if TYPE_CHECKING assertion blocks.

Validation run:

• uv run ruff check src tests/test_protocol_abstract_contracts.py
• uv run pytest tests/test_protocol_abstract_contracts.py tests/test_protocols_exports.py tests/graph_engine/test_response_coordinator.py tests/nodes/test_human_input_runtime_binding.py tests/nodes/human_input/test_human_input_node.py tests/nodes/human_input/test_human_input_node_internal.py

Checklist

• This pull request links the issue it resolves or advances
• This pull request title follows Conventional Commits, and any breaking change is marked with !
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation

[QuantumGhost]

The massive if TYPE_CHECKING statements are ugly. I'll split static assertions to dedicated files and submit another PR.

[QuantumGhost]

superseded by #138