Phase 0: test scaffolding + golden fixtures

- ruff config, offscreen smoke tests for all 7 GUI apps/panels
- golden v6 .sras fixtures (complete + 4 truncation variants) generated
  by the pre-refactor writer, with expected header/frontier JSON
- golden geometry fixtures from the pre-refactor _build_scan_params
- consistency tests proving current code reproduces the goldens

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

ruff.toml

@@ -0,0 +1,19 @@
+target-version = "py311"
+line-length = 120
+
+[lint]
+# F: pyflakes (unused imports/variables, undefined names)
+# E7/E9: comparison and runtime-error prone constructs
+# B: bugbear (mutable defaults, useless expressions)
+select = ["F", "E7", "E9", "B"]
+ignore = [
+    "E731",  # lambda assignment — used deliberately for short Qt slot glue
+    "E741",  # ambiguous single-letter names — used in math-heavy geometry code
+]
+
+[lint.per-file-ignores]
+# Wildcard re-exports are part of this package's (legacy) public surface
+# until Phase 1 empties it.
+"hardware/__init__.py" = ["F401", "F403"]
+"hardware/pybbd202/__init__.py" = ["F401", "F403"]
+"scanning/__init__.py" = ["F401", "F403"]