Enforce the documented coverage threshold in CI #79

Problem

The repository configures coverage collection and documents a local report, but CI and release qualification run pytest without coverage and pyproject.toml defines no enforced minimum. The ecosystem standard requires the test lane to enforce its coverage threshold.

Acceptance criteria

• pyproject.toml defines a conservative threshold supported by a fresh full-suite run.
• Pull-request CI and release quality collect package coverage and fail below that threshold.
• Contributor and agent verification commands match the enforced lane.
• A focused test proves the configured threshold is active.
• Ruff, ty, tests, resource warnings, strict docs, build, actionlint, and pre-commit pass.