Generate an external feature-gap report for skipped Test262 features

[dowdiness]

Context

A review of another MoonBit JavaScript runtime's QuickJS-aligned Test262 feature configuration found that it lists many features that this repo currently skips in scripts/test262_skip_metadata.py.

This should be used as a planning signal only, not as a conformance comparison. Do not copy external source code without resolving licensing.

Proposal

Add a small script/report that compares this repo's authoritative skip metadata against an external Test262 feature config, initially tested with a small fixture config.

Tasks

• Parse a Test262 config [features] section into supported vs skipped feature names.
• Compare that set with SKIP_FEATURES / SKIP_FLAGS from scripts/test262_skip_metadata.py.
• Print a Markdown report with:
  • features we skip that the external config runs,
  • features both configs skip,
  • features that need manual classification.
• Keep the script non-authoritative: it must not report pass rates or claim conformance.
• Add tests with a small fixture config rather than requiring a network clone.

Acceptance criteria

• moon check && moon test still pass.
• Python tests cover the parser and comparison behavior.
• Documentation explicitly says runner results from CI remain authoritative for this repo.

[dowdiness]

Implemented in PR #460: scripts/test262_feature_gap.py parses a Test262 config's [features] section and compares it against scripts/test262_skip_metadata.json. Produces a Markdown report (features we skip / both skip / both run) with a non-authoritative disclaimer. scripts/test_test262_feature_gap.py has 15 Python unit tests (all passing). make test262-feature-gap-test runs them. docs/TEST262.md documents usage. All acceptance criteria met.