test: cross-binding parity gaps — type_mismatch (napi) + @extends merged-FM (python)

[dean0x]

Deferred (pre-existing) finding from the PR #161 code review — surfaced by the python reviewer. The two binding parity suites have drifted. Two follow-up gaps:

(a) napi parity: mds::type_mismatch

PR #161 added a mds::type_mismatch (#152) error-parity case to the Python suite (crates/mds-python/tests/test_parity.py), but the napi __test__ suite still lacks the equivalent case. Add the mds::type_mismatch case to the napi parity suite.

(b) python parity: @extends merged frontmatter

The Python binding has no parity coverage for #154 @extends deep-merged frontmatter because it lacks compile_virtual plumbing. Add compile_virtual to the Python binding and an @extends merged-FM parity case.

Notes

• A related but distinct napi test gap (string-source basePath import resolution in Windows CI) is tracked in test(napi): exercise string-source basePath import resolution in Windows CI #148.