Housekeeping: extend .gitignore, drop stale v2-v5 format docs

SRAS_FORMAT.md/.html documented only v2-v5 and were superseded by
scan_format.md in July. Ignore .DS_Store, screenshots, pytest cache,
and test scratch outputs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -1,2 +1,8 @@
 __pycache__/
 .venv/
+.DS_Store
+.pytest_cache/
+*.png
+*.sras
+baseline*.txt
+after*.txt