Harden Protocol contracts with abstract methods and regression checks

[QuantumGhost]

AI Disclosure: This issue is generated with Codex and GPT-5.5.

Summary

Some Protocol definitions in this repository declare members without @abstractmethod, which can allow accidental concrete subclasses in multi-level inheritance chains.

Problem

Without consistently marking protocol members as abstract, a subclass can unintentionally become instantiable after only partially overriding inherited members. This weakens interface contracts and can hide missing method implementations until runtime behavior diverges.

Proposed changes

1. Add @abstractmethod to protocol members that currently lack it.
2. Add static implementation assertions (in sibling files with _type_contract suffix) for in-repo protocol implementations, following the _assert_readonly_variable_pool pattern.
3. Add regression tests to ensure protocol contracts stay abstract and are not unintentionally implemented without explicit overrides.

Validation target

• New tests should prove that:
  • protocol members remain abstract,
  • direct subclasses without overrides cannot be instantiated,
  • indirect partial overrides remain abstract.

[QuantumGhost]

Implemented in draft PR #138: #138

Summary:

• Protocol methods are marked abstract.
• In-repository protocol implementations have colocated static type contracts.
• Regression tests cover indirect nominal implementations without overrides.
• Static type contract modules are excluded from published distributions and documented in CONTRIBUTING.md.

Validation:

• make check
• uv run pytest
• uv build, followed by wheel/sdist inspection for *_type_contract.py exclusions