feat: consolidate check-tests-structure hook into this repository

[leoschwarz]

check-tests-structure (tests tree must mirror sources tree) lived in a personal GitHub account, while depiction and bfabricPy depend on it. This moves the tool here and publishes it as a pre-commit hook so those projects can drop that dependency.

Key changes

• .ci/check-tests-structure/ — the tool vendored from leoschwarz/check-tests-structure@1228f93 (incl. its unreleased allow_missing_* fix) as a standalone subproject with its own pyproject.toml, uv.lock and MIT license. Distribution renamed to fgcz-check-tests-structure; import package and console script names unchanged, so consumers are drop-in.
• .pre-commit-hooks.yaml — the consumer-facing surface.
• The hook runs on this repo in the pre-commit stage; the subproject's own tests run in the pre-push stage.
• Root gates: Ruff lints/formats .ci/** under a relaxed rule set, deptry excludes it, Pyright and coverage already did.

Non-obvious bits

• pre-commit's language: python installs the hook repo's root package, so a subdirectory tool normally cannot be consumed through the plain repo: form. additional_dependencies: ["./.ci/check-tests-structure"] resolves relative to the cloned hook repo, which is what makes it work.
• The hook environment therefore also installs this repo's root package, which requires Python >= 3.12. docs/check-tests-structure.md carries a git+...#subdirectory= fallback for older hook environments.
• Ported as-is, so sources_path/tests_path still resolve against the working directory rather than the declaring pyproject.toml; workspace members must be checked from their own directory.
• Consumers pin rev:, so this needs a release tag after merge.

Verification

Both hook stages pass; the vendored suite (12 tests) passes; pre-commit try-repo passes on a matching tree and exits 1 with fuzzy suggestions on a missing/orphan test; the git+...#subdirectory= form was installed and run from a git URL.

Consumer repos are untouched — docs/check-tests-structure.md has the pre-commit and nox snippets.

🤖 Prepared with assistance from Claude Opus 5 via Claude Code.