Add CI gate: LANGUAGE_REGISTRY ↔ NATIVE_SUPPORTED_EXTENSIONS consistency

[carlos-alm]

Background

Follow-up to #1071. The 11 WASM-only language extractors have all been ported to Rust, and a drift guard exists between the JS-side NATIVE_SUPPORTED_EXTENSIONS Set and Rust's parser_registry.rs::from_extension (see tests/parsers/native-drop-classification.test.ts — the NATIVE_SUPPORTED_EXTENSIONS drift guard describe block).

However, the gate is one layer removed from the literal acceptance criterion in #1071:

A CI gate prevents future drift: a test that fails when a JS LANGUAGE_REGISTRY entry has no corresponding Rust extractor (or an explicit allowlist of intentionally WASM-only languages).

The gap

NATIVE_SUPPORTED_EXTENSIONS in src/domain/parser.ts:434 is hand-maintained, not derived from LANGUAGE_REGISTRY. The chain looks like this:

• LANGUAGE_REGISTRY (JS source of truth for supported languages, drives WASM parsing)
• NATIVE_SUPPORTED_EXTENSIONS (JS-side hand-maintained mirror of the Rust enum)
• parser_registry.rs::from_extension (Rust source of truth for native extractor coverage)

The existing drift guard covers links 2 ↔ 3. Link 1 ↔ 2 has no test. If someone adds a new WASM-only language to LANGUAGE_REGISTRY without also adding the extensions to NATIVE_SUPPORTED_EXTENSIONS and without porting a Rust extractor, no test fires — those files would silently be classified as unsupported-by-native on the native engine and only the JS WASM backfill (#967, #1068) would parse them. That's exactly the regression class #1071 was meant to prevent.

Proposal

Add a drift test that asserts every extension in LANGUAGE_REGISTRY is present in NATIVE_SUPPORTED_EXTENSIONS, with an explicit WASM_ONLY_ALLOWLIST for any intentional exceptions (currently empty). When the allowlist is empty, the test is equivalent to LANGUAGE_REGISTRY ↔ Rust extractor parity.

Suggested location: extend tests/parsers/native-drop-classification.test.ts with a new describe block, e.g. LANGUAGE_REGISTRY ↔ NATIVE_SUPPORTED_EXTENSIONS parity.

Acceptance criteria

• New test fails if a LANGUAGE_REGISTRY entry's extensions are missing from NATIVE_SUPPORTED_EXTENSIONS (and not on the allowlist).
• Existing tests still pass on main without modification.
• The allowlist is documented in-code so future contributors know when to use it vs. when to port to Rust.