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>

tests/golden/sras_expected.json

@@ -0,0 +1,170 @@
+{
+  "header": {
+    "version": 6,
+    "n_angles": 2,
+    "x_start_nominal": 1.0,
+    "y_start_nominal": 1.0,
+    "x_delta_nominal": 0.019999999552965164,
+    "y_delta_nominal": 0.019999999552965164,
+    "row_spacing": 0.009999999776482582,
+    "velocity": 100.0,
+    "laser_freq": 20000.0,
+    "samples_per_frame": 8,
+    "sample_rate": 6250000000.0,
+    "bytes_per_sample": 1,
+    "n_channels": 3,
+    "angles": [
+      0.0,
+      -180.0
+    ],
+    "per_angle": [
+      {
+        "angle": 0.0,
+        "x_start": 1.0,
+        "x_delta": 0.019999999552965164,
+        "n_frames": 4,
+        "n_rows": 3,
+        "y_positions": [
+          1.0,
+          1.0099999904632568,
+          1.0199999809265137
+        ]
+      },
+      {
+        "angle": -180.0,
+        "x_start": 1.0,
+        "x_delta": 0.019999999552965164,
+        "n_frames": 4,
+        "n_rows": 3,
+        "y_positions": [
+          1.0,
+          1.0099999904632568,
+          1.0199999809265137
+        ]
+      }
+    ],
+    "data_start_offset": 265
+  },
+  "statuses": {
+    "complete.sras": [
+      {
+        "index": 0,
+        "angle_deg": 0.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 3,
+        "status": "OK"
+      },
+      {
+        "index": 1,
+        "angle_deg": -180.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 553,
+        "n_rows_available": 3,
+        "status": "OK"
+      }
+    ],
+    "trunc_midrow_a1.sras": [
+      {
+        "index": 0,
+        "angle_deg": 0.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 3,
+        "status": "OK"
+      },
+      {
+        "index": 1,
+        "angle_deg": -180.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 553,
+        "n_rows_available": 1,
+        "status": "TRUNCATED"
+      }
+    ],
+    "trunc_rowboundary_a1.sras": [
+      {
+        "index": 0,
+        "angle_deg": 0.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 3,
+        "status": "OK"
+      },
+      {
+        "index": 1,
+        "angle_deg": -180.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 553,
+        "n_rows_available": 2,
+        "status": "TRUNCATED"
+      }
+    ],
+    "trunc_angleboundary.sras": [
+      {
+        "index": 0,
+        "angle_deg": 0.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 3,
+        "status": "OK"
+      },
+      {
+        "index": 1,
+        "angle_deg": -180.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 553,
+        "n_rows_available": 0,
+        "status": "MISSING"
+      }
+    ],
+    "trunc_midrow_a0.sras": [
+      {
+        "index": 0,
+        "angle_deg": 0.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 1,
+        "status": "TRUNCATED"
+      },
+      {
+        "index": 1,
+        "angle_deg": -180.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 0,
+        "status": "MISSING"
+      }
+    ],
+    "header_only.sras": [
+      {
+        "index": 0,
+        "angle_deg": 0.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 0,
+        "status": "MISSING"
+      },
+      {
+        "index": 1,
+        "angle_deg": -180.0,
+        "n_rows": 3,
+        "row_bytes": 96,
+        "data_offset": 265,
+        "n_rows_available": 0,
+        "status": "MISSING"
+      }
+    ]
+  }
+}