feat(Q2): heatmap generation with bbox accumulation and colormap

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_heatmap.py

@@ -0,0 +1,17 @@
+import numpy as np
+from vigilar.detection.heatmap import accumulate_detections, render_heatmap_png
+
+def test_accumulate_detections_returns_grid():
+    bboxes = [[0.5, 0.5, 0.1, 0.1], [0.5, 0.5, 0.1, 0.1], [0.2, 0.3, 0.1, 0.1]]
+    grid = accumulate_detections(bboxes, grid_w=64, grid_h=36)
+    assert grid.shape == (36, 64)
+    assert grid.sum() > 0
+    assert grid[18, 32] > grid[0, 0]  # center of [0.5, 0.5]
+
+def test_generate_heatmap_returns_png_bytes():
+    bboxes = [[0.3, 0.4, 0.1, 0.1]] * 20
+    grid = accumulate_detections(bboxes)
+    frame = np.zeros((360, 640, 3), dtype=np.uint8)
+    png_bytes = render_heatmap_png(grid, frame)
+    assert isinstance(png_bytes, bytes)
+    assert png_bytes[:4] == b"\x89PNG"