Restore CSS property metadata generator

[wieslawsoltes]

Current main invokes the CSS property metadata generator from CI, but #309 omitted the generator file, so every Linux CI run fails before compiling the proposed change. This restores the deterministic generator expected by the checked-in header and adds focused negative coverage for schema, masks, targets, expansion arity, and stale output.

Closes #311

Validation

• python3 -m unittest scripts/test_css_property_metadata.py -v — 6/6 pass
• python3 tools/generate_css_property_metadata.py --check — exact checked-in output
• python3 -m py_compile tools/generate_css_property_metadata.py scripts/test_css_property_metadata.py
• git diff --check

This changes generation and CI validation only; it does not change runtime CSS behavior.

[wieslawsoltes]

Superseded by the concurrently merged #313, which added tools/generate_css_property_metadata.py and expanded the shared metadata generator. I verified current main locally: the focused generator suite passes 6/6 and --check succeeds. Closing this duplicate branch without merging.