test(depth_align): real z-buffer collision + drop dead var

The 2× focal-length collision test wasn't actually colliding (adjacent IR
pixels rounded to different RGB columns). 4× makes them collapse, so the
np.minimum.at reduce is now exercised. Also drop unused H_ir, W_ir.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/presa_mano/perception/depth_align.py

@@ -45,7 +45,6 @@ def align_depth_to_rgb(
         )
 
     H_rgb, W_rgb = reg.rgb_height, reg.rgb_width
-    H_ir, W_ir = reg.ir_height, reg.ir_width
 
     valid = depth_ir_mm > 0
     if not valid.any():

tests/test_depth_align.py

@@ -57,15 +57,15 @@ def test_sparse_pixels_preserved():
 def test_z_buffer_keeps_closer_point():
     """When two IR pixels collide on the same RGB pixel, the smaller depth wins.
 
-    Force a collision with K_ir = 2 × K_rgb (same resolution), so adjacent IR
+    Force a collision with K_ir = 4 × K_rgb (same resolution), so adjacent IR
     pixels round to the same RGB pixel."""
     W = H = 16
     K_rgb = np.array([[60.0, 0.0, W / 2.0],
                       [0.0,  60.0, H / 2.0],
                       [0.0,  0.0,  1.0]], dtype=np.float64)
     K_ir = K_rgb.copy()
-    K_ir[0, 0] *= 2.0   # double focal length → two IR pixels collapse to one RGB pixel
-    K_ir[1, 1] *= 2.0
+    K_ir[0, 0] *= 4.0   # 4× focal length → adjacent IR pixels collapse to one RGB pixel
+    K_ir[1, 1] *= 4.0
     reg = CameraRegistration(
         K_rgb=K_rgb, K_ir=K_ir,
         R_ir_to_rgb=np.eye(3, dtype=np.float64),
@@ -73,16 +73,13 @@ def test_z_buffer_keeps_closer_point():
         rgb_width=W, rgb_height=H, ir_width=W, ir_height=H,
     )
     depth_ir = np.zeros((H, W), dtype=np.uint16)
-    # Two IR pixels (5, 5) and (6, 5) both project to the same RGB pixel near (4, 5)
-    # (when projected through K_ir with depth 1m and re-projected through K_rgb).
-    depth_ir[5, 5] = 1500
-    depth_ir[5, 6] = 800
+    # IR pixels (8, 8) and (8, 9) both project to RGB pixel (8, 8):
+    # u_rgb = 0.25*(u_ir - 8) + 8 → 8.0 and 8.25 (both round to 8).
+    depth_ir[8, 8] = 1500
+    depth_ir[8, 9] = 800
     depth_rgb = align_depth_to_rgb(depth_ir, reg)
     nonzero = depth_rgb[depth_rgb > 0]
-    # At least one RGB pixel must receive the closer (800) value; nothing further
-    # may be ≥ 1500 if a collision actually occurred. If they didn't collide, we
-    # see exactly two values [800, 1500]; if they did, we see only [800].
-    assert nonzero.size in (1, 2)
-    assert int(nonzero.min()) == 800
-    if nonzero.size == 1:
-        assert int(nonzero[0]) == 800   # collision happened, closer point won
+    # Exactly one filled pixel; z-buffer kept the closer (800) value.
+    assert nonzero.size == 1
+    assert int(nonzero[0]) == 800
+    assert int(depth_rgb[8, 8]) == 800