test: add preprocessor drift fixtures

[rlippmann]

What changed

• Added 10 shared preprocessor heuristic fixtures under tests/fixtures/preprocessor/ to cover regular-test-only Python cases.
• Added fixture coverage for mixed-intent, discourse-marker/meta-prefix, near-miss, modal-prefix, and no-directive question behavior currently asserted in Python tests.
• Included the confirmed drift case: use docker, actually prohibit docker -> unknown.

Why

• Shared preprocessor conformance fixtures had drifted behind Python regular tests for several explicitly asserted behaviors.
• These fixture additions make those behaviors part of shared source-of-truth fixture coverage, reducing parity gaps across ports.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)