Generate effective CSS property metadata

[danwalmsley]

Summary

• replace duplicated effective-longhand/property-mask conditionals with a generated 61-entry metadata registry
• share the registry between cascade rollback, property masks, and the six logical shorthands that require ordinary application expansion
• fix two-value logical inset and border width/color behavior across stylesheet rollback and inline CSSOM recascade
• add a generator stale-output gate, registry invariants, a reproducible lookup benchmark, and validation evidence

Performance boundary

High-frequency direct fields still return before metadata lookup. Common margin/padding/gap/overflow shorthands retain their existing direct application paths. Only six previously incorrect logical shorthands expand during ordinary application; rollback continues to expand only on the rare rollback path.

Identical AppleClang 21 -O3 A/B over 24 million mixed property-mask lookups: 502.446 ms baseline median versus 424.271 ms candidate median (-15.56%).

Validation

• Chrome 153 effective-property contract: 15/15
• native current parser: 15/15
• native legacy parser: 15/15
• unchanged cascade winner matrix: 17/17
• adjacent CSSOM contracts: 21/21
• NativeWeb CSS service/shared styles/compiler: 3/3
• generator tests: 3/3 plus stale-output check
• cascade-layer exact work counters invariant as unrelated nodes grow 32 -> 1024

This is the property-metadata stage of #235. It intentionally does not touch #238 Linux timing attribution or #297 stylesheet-finalization work.