fix(cp5): close PNG figures on error + force Agg + _title unit tests

proteus/viz/png.py

@@ -36,7 +36,7 @@ def write_pngs(
     """Render every frame to `<out_dir>/frame_NNN.png`; return the paths in order."""
     import matplotlib
 
-    matplotlib.use("Agg")  # headless: no window, no display required
+    matplotlib.use("Agg", force=True)  # headless; force switch even if a GUI backend pre-loaded pyplot via the engine
     import matplotlib.pyplot as plt
 
     out_dir = Path(out_dir)
@@ -47,12 +47,14 @@ def write_pngs(
         #   scale: int = 4, color_map: Optional[dict] = None) -> ndarray
         # The first `steps` arg is a display counter (ignored inside the function).
         rgb = frame_to_rgb_array(0, step.frame, scale, COLOR_MAP)
-        fig, ax = plt.subplots(figsize=(4.0, 4.4))
-        ax.imshow(rgb, interpolation="nearest")
-        ax.axis("off")
-        ax.set_title(_title(step), fontsize=9)
         path = out_dir / f"frame_{i:03d}.png"
-        fig.savefig(path, dpi=100, bbox_inches="tight")
-        plt.close(fig)
+        fig, ax = plt.subplots(figsize=(4.0, 4.4))
+        try:
+            ax.imshow(rgb, interpolation="nearest")
+            ax.axis("off")
+            ax.set_title(_title(step), fontsize=9)
+            fig.savefig(path, dpi=100, bbox_inches="tight")
+        finally:
+            plt.close(fig)
         paths.append(path)
     return paths

tests/viz/test_png.py

@@ -34,3 +34,29 @@ def test_write_pngs_creates_missing_dir(tmp_path):
     paths = write_pngs(steps, nested)
     assert nested.is_dir()
     assert len(paths) == len(steps)
+
+
+from proteus.viz.png import _title
+from proteus.viz.reconstruct import FrameMeta, FrameStep
+import numpy as np
+
+
+def _step(meta):
+    return FrameStep(np.zeros((2, 2), dtype=int), meta)
+
+
+def test_title_cut_phase():
+    assert _title(_step(FrameMeta(phase="cut", index=1))) == "Cut 1 (pre-roll)"
+
+
+def test_title_play_congruent_no_terminal():
+    meta = FrameMeta(phase="play", index=2, turn_idx=2, action="up", was_congruent=True)
+    assert _title(_step(meta)) == "Turn 2: action=up ✓"
+
+
+def test_title_play_diverged_with_terminal():
+    meta = FrameMeta(
+        phase="play", index=3, turn_idx=3, action="down",
+        was_congruent=False, terminal="survived",
+    )
+    assert _title(_step(meta)) == "Turn 3: action=down ✗  [survived]"