chore: mark methods in Protocol definitions as abstract

[QuantumGhost]

AI disclosure: This PR was primarily drafted with Codex using GPT-5.5.
I have reviewed and edited the final diff, and I am responsible for the content.

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Related Issue

Closes #136

Summary

This PR hardens protocol contracts by:

1. Preventing nominal subclasses from inadvertently becoming instantiable unless they explicitly override the protocol methods.

Changes included:

• Marks protocol methods as abstract.
• Adds regression tests for abstract protocol methods.

Validation performed:

• make check
• uv run pytest
• uv build --out-dir /tmp/graphon-dist.FoJno3

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]

Adjusted the static type contract scope based on review feedback.

Changes:

• Removed files for implementations that already directly subclass their target Protocol, such as .
• Kept structural contracts where the implementation does not nominally inherit the target Protocol.
• For mixed files, retained only the structural assertions.
• Updated CONTRIBUTING.md to document that type contract files are for structural protocol implementations.

Validation:

• make check
• uv run pytest