fix(ci): treat sdist VCS-ignore files as optional in distribution check

[QuiteYellow]

The Validate workflow's Package artifacts job started failing on main right after #21 merged:

Run python tools/check_distribution.py dist
distribution check failed
Error: Process completed with exit code 1.

Cause

check_sdist compared the sdist member list against an exact expected set that hardcoded .hgignore. hatchling bundles the VCS ignore files it finds, but which ones ship depends on the hatchling version — 1.31.0 (what CI resolved) ships only .gitignore in the sdist, while older releases also included .hgignore. #21 was validated against an older hatchling, so the exact-match check passed locally and failed in CI.

The wheel check was unaffected.

Fix

Split the sdist expectation into a required set (tracked sources + LICENSE, PKG-INFO, README.md, pyproject.toml, _version.py) and an optional set (.gitignore, .hgignore). The check now asserts required <= relative <= required | optional, so it passes whether hatchling emits one ignore file, both, or neither, while still catching any genuinely unexpected member.

Validation

• python -m build + python tools/check_distribution.py dist → distribution contents verified (hatchling 1.31.0, Python 3.14)
• pytest -q → 52 passed

[QuiteYellow]

@Jason-Morcos heads up: the Package artifacts job from #21 went red on main once CI resolved hatchling 1.31.0, which stopped shipping .hgignore in the sdist. #27 loosens the sdist check to tolerate that. Shouldn't affect #23–#25.