test(regexp): retire nested-alternation Prism skips

[pmatos]

Summary

• add spec-derived regression coverage for the reported parenthesis and Bison brace match-priority patterns
• remove the three JSSE-only PrismJS fixture skips so JSSE and Node execute the same 2,563 fixtures
• update the library-harness documentation to reflect full PrismJS coverage
• preserve the current RegExp backend because the current main branch already matches the required ECMAScript ordering for both reproductions

Validation

• uv run python scripts/run-custom-tests.py test262-extra/RegExp-nested-alternation-priority.js (1/1)
• uv run python scripts/run-custom-tests.py (7/7)
• uv run python scripts/run-test262.py test262/test/built-ins/RegExp/ (3,756/3,756)
• ./scripts/run-library-tests.sh prismjs --clean (JSSE and Node: 2,563/2,563)
• cargo fmt --check
• cargo clippy
• ./scripts/lint.sh
• cargo build --release
• cargo test --release (294 unit tests plus smoke oracle)
• TZ=UTC uv run python scripts/run-test262.py (99,568/99,568; zero regressions)

The ambient Europe/Berlin full-suite run exposed 22 unrelated default-time-zone Intl failures; the repository baseline was last verified under UTC, and the affected Intl subset passes 328/328 under TZ=UTC.

Closes #271

[github-actions]

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