Add effective capability parity contract

[RobLe3]

Summary

• add public, binding-neutral effective service capability types, parser, precedence resolver and matcher
• pin the exact shared v1 schemas and parity fixture
• preserve complete same-intent variants and portable refusal semantics without changing discovery or dispatch

Evidence

• scripts/run_sdk_quality.py passed on Python 3.11, 3.12 and 3.13
• 75.69% line coverage (75% floor)
• Ruff, mypy baseline, dependency audit, locked build and clean-install smoke passed

Compatibility

The feature is additive and unused by existing calls, provider execution, registration and streaming paths. Heuristic evidence is accepted only when explicitly labelled and only when stronger evidence is absent. No package was published or deployed.

Closes #91