Repair shell detection and complete core parser and finder coverage

[emoriwan]

Closes #75

[pirog]

@emoriwan can you tell me why the unit tests are failing and propose a fix?

[emoriwan]

@pirog

All three OS jobs fail on the same incorrect assertion in test/parse.spec.js:95: it expects eight tests.test entries, but there are seven.

parse() defaults to testHeader: ['Test'], so basic-example.md’s Verifying section correctly lands in tests.invalid; its test is the assumed eighth entry. The other 39 unit tests pass, as do lint and every shell, Leia, and module-format job. The Browserslist warning is unrelated.

Proposed fix: leave parser behavior unchanged. Replace the brittle aggregate count with assertions that commands from both code blocks appear in the test bucket. For maximum isolation, use a tiny fixture containing one Testing section with two code blocks and assert both parsed commands.

[pirog]

@emoriwan ok, can you do that and push up the fix for me to look at pls?

[emoriwan]

@pirog

I received your comment, but I couldn't safely publish the detailed response. I've kept it in the linked private session for review.