Implement effective CSS property cascade semantics

[danwalmsley]

Summary

• evaluate rollback over effective longhands for common box shorthands while preserving the ordinary no-rollback hot path
• preserve authored inline declaration order and replay overlapping shorthand/longhand declarations correctly
• implement CSSOM all replacement/removal and important all: unset behavior
• add a 12-case browser-referenced WPT contract, native scaling regression, and validation evidence

Validation

• Chrome 153: 12/12 contract subtests
• native current parser: 12/12
• native legacy parser: 12/12
• existing cascade winner matrix: 17/17
• adjacent CSSOM contracts: 16/16
• NativeWeb CTest: 3/3
• cascade-layer scaling counters invariant with 32 vs 1024 unrelated nodes

Closes a bounded stage of #235; the issue remains open for complex shorthands, broader origins/inheritance, and remaining structural work.