tech-debt: Parse WASM return shape at FFI boundary

[dean0x]

Problem

validateWasmShape (packages/mds/src/backend/wasm.ts:125-135) checks that required exports exist (compile, check, compileMessages) but does not validate the shape of return values crossing the WASM FFI boundary.

Current Behavior

Checks presence of export names only:

• if (!wasm.compile || !wasm.check || !wasm.compileMessages)

Does not validate:

• Return type structure (e.g., CompileResult shape)
• Presence of required fields (e.g., .code, .messages)
• JSON schema compliance

Root Cause

This is a pre-existing pattern — both compile and check functions have the same validation gap. Not introduced by #87 (compileMessages added), but worth addressing as part of a broader boundary-hardening pass.

Solution

Extend validateWasmShape to parse and validate return shapes:

• Call each export with minimal/empty input
• Assert return structure matches CompileResult / CompileMessagesResult shape
• May be combined with other WASM boundary validations

Priority

Low — Correctness is ensured at call sites; this adds parse-don't-validate rigor at the boundary.

Related

• Future boundary-hardening pass
• See also feat: structured JSON output via @message blocks and compileMessages() (#56) #87 resolution summary (B11)