docs(gui): document the new GUI test layer

REQUIREMENTS §16 updates the test count (1777 → 1916) and breaks out
the GUI subset. DEVELOPER's Tests section gains the 'gui' marker
recipes and the new tests/gui/ tree under test layout, plus a short
'GUI test layer' explainer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/REQUIREMENTS.md

@@ -174,7 +174,12 @@ and proceeds.
 - **Dev**: pytest, tox.
 
 ## 16. Test coverage
-- 1,777 tests passing, 0 skipped, 0 xfailed (incl. 15 perf-shape regression tests).
+- 1,916 tests passing, 0 skipped, 0 xfailed.
+  - 1,777 core + CLI tests (run with `pytest -m 'not gui'` for a quick loop).
+  - 139 GUI tests under `tests/gui/` driving Streamlit pages via `AppTest`
+    (smoke + EN/ES localization, chrome, gate, workflows, dedup review,
+    advanced panels, error paths, findings panel). Marked `gui`.
+  - Includes 15 perf-shape regression tests.
 - Fixture corpora: text-cleaner (21), encodings (31), reference UTF-8 (9), format-cleaner (199 buyer cases + 20-row international stress fixture), missing-handler (3 use cases + 16 edge cases), column-mapper (3 use cases + 5 edge cases).
 - Run: `python run_tests.py [--tool …] [--fixtures] [--coverage]`.