Restore omitted CSS property metadata generator and CI contract

[wieslawsoltes]

Problem

Current main (26431500) runs scripts/test_css_property_metadata.py and python tools/generate_css_property_metadata.py --check from Linux CI, but the generator file was not included when the generated CSS property metadata work merged in #309.

The Linux job therefore fails before compiling any PR changes:

FileNotFoundError: tools/generate_css_property_metadata.py

This currently blocks unrelated PRs, including #310. The checked-in generated header identifies that exact generator as its source, and the validation document says CI validates both its unit tests and --check mode.

Scope

• Add the missing deterministic generator at tools/generate_css_property_metadata.py.
• Validate catalog version, schema, property names, mask symbols, duplicate names, aliases, expansion targets and maximum expansion arity.
• Generate the existing sorted effective_property_metadata_catalog byte-for-byte.
• Make --check fail clearly when the checked-in header is stale.
• Keep generation locale independent and deterministic.

Acceptance gates

• python -m unittest scripts/test_css_property_metadata.py -v
• python tools/generate_css_property_metadata.py --check
• Generated output matches the header already reviewed in Generate effective CSS property metadata #309 byte-for-byte.
• Negative tests cover duplicate names, unknown aliases/targets, invalid masks/schema/arity, and stale output.
• The focused Linux CI failure is resolved without changing runtime CSS behavior.

Relationship

This is a focused repair under the CSS epic #235. It restores the generator and CI contract intended by #309; it does not change the CSS runtime or the active #297 stylesheet-publication optimization.

[wieslawsoltes]

Resolved by the concurrently merged #313. Verified on current main b69f931d: python3 -m unittest scripts/test_css_property_metadata.py -v passes 6/6 and python3 tools/generate_css_property_metadata.py --check succeeds. The duplicate repair PR #314 was closed without merging.