Audit and promote skipped test262 cases

Scope

• Inventory every test262 skip that remains outside the configured runner denominator.
• Classify each skipped case by exact path and reason from conformance/test262.zig or harness loading behavior.
• Promote any case/category that is already supported, with focused worker proof before changing the runner.

Exit criteria

• A committed machine-readable skipped-case inventory exists.
• The docs distinguish scored tests from skipped tests using that inventory.
• Any easy supported skips are promoted and verified.
• Remaining skips have concrete blockers, not vague categories.

Notes

• Current verified configured result: VALID 48368/48368, NEGATIVE 4669/4669, host-fail 0, skipped 140.
• Do not claim skipped cases are implemented until they pass after promotion.

Implemented in ec91da9. Added --list-skips and committed docs/.data/test262-skips.tsv with 139 remaining skipped cases: 94 module+async/top-level-await harness, 35 tail-call-optimization, 10 unsupported SpiderMonkey staging paths. Promoted 1 case from skipped to passing: test/staging/sm/String/string-upper-lower-mapping.js, verified with focused worker test/staging 1309:p and full saved parent run VALID 48369/48369, NEGATIVE 4669/4669, skipped 139.