Publish test262 conformance for the vendored Boa engine in CI #35

Summary

tests/test262/ exists in the repo but contains only __pycache__; there is no committed runner output, CI job, or published conformance number. The vendored Boa in vendor/boa_engine has been modified (inline caches, SLICEBC CodeBlock precompilation, fast paths), so upstream Boa's published ~90 % figure no longer describes this engine.

Why it matters

Users migrating from Node/Bun/Deno (all ≥ 99 % test262) need a number, and the project needs a regression guard for its engine patches. A conformance score is also the single most credible counter to "it's an interpreter, how much of JS does it actually run?".

Proposal

• Add tests/test262/run.py (or reuse what generated the __pycache__) that runs the official tc39/test262 suite against slice run with the --deterministic flag, honouring includes, flags: [onlyStrict|noStrict|module|async], and negative expectations.
• Commit an excludelist with reasons (e.g. Intl disabled, Temporal not implemented) rather than silently skipping.
• Publish the result in CI as a percentage per directory (built-ins/, language/, intl402/, annexB/) and fail the job on regression.
• Surface the headline number in the README next to the benchmark table.
• Track the vendored Boa delta: a vendor/boa_engine/SLICE_PATCHES.md listing each modification and whether it affects semantics, so the test262 diff against upstream Boa 0.22 is explainable.