Golden integration test harness

[gavinbee]

Wire `pytest -m integration` to run the parser against real-scan fixtures and compare against REMS golden sets.

• Reads from `tests/fixtures/golden/-.json` and matched PDFs in `data/`.
• Skipped by default; only runs with `-m integration`.
• Asserts (official, position, successful) tuples match the golden set, modulo position-name normalization.
• Document the rems-sync golden-set generation flow in `docs/troubleshooting.md` (or a new `docs/testing.md`).

Depends on #10.