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():