feat(perception): vectorised IR→RGB depth alignment

Forward-project IR depth pixels into the RGB pixel grid with a z-buffer.
Output uint16 mm at RGB resolution, ready to feed SAM21.

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

tests/test_depth_align.py

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+"""Unit tests for align_depth_to_rgb."""
+from __future__ import annotations
+
+import numpy as np
+
+from presa_mano.perception.depth_align import align_depth_to_rgb
+from presa_mano.perception.depth_to_rgb import CameraRegistration
+
+
+def _identity_reg(W: int = 64, H: int = 48) -> CameraRegistration:
+    """Same K for RGB and IR, no extrinsic offset."""
+    K = 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)
+    return CameraRegistration(
+        K_rgb=K.copy(), K_ir=K.copy(),
+        R_ir_to_rgb=np.eye(3, dtype=np.float64),
+        t_ir_to_rgb=np.zeros(3, dtype=np.float64),
+        rgb_width=W, rgb_height=H,
+        ir_width=W, ir_height=H,
+    )
+
+
+def test_identity_alignment_preserves_depth():
+    """K_ir == K_rgb and no extrinsic → every IR pixel maps to itself."""
+    reg = _identity_reg()
+    depth_ir = np.full((reg.ir_height, reg.ir_width), 1000, dtype=np.uint16)
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    assert depth_rgb.shape == (reg.rgb_height, reg.rgb_width)
+    assert depth_rgb.dtype == np.uint16
+    nonzero = depth_rgb[depth_rgb > 0]
+    # Coverage close to full (rounding can drop a few border pixels).
+    assert nonzero.size >= 0.95 * depth_rgb.size, f"coverage too low: {nonzero.size}/{depth_rgb.size}"
+    # All filled pixels equal the input depth (±1 mm rounding).
+    assert np.all(np.abs(nonzero.astype(int) - 1000) <= 1)
+
+
+def test_zero_depth_pixels_stay_zero():
+    """IR pixels with depth=0 must not produce any RGB output."""
+    reg = _identity_reg()
+    depth_ir = np.zeros((reg.ir_height, reg.ir_width), dtype=np.uint16)
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    assert depth_rgb.shape == (reg.rgb_height, reg.rgb_width)
+    assert int(depth_rgb.sum()) == 0
+
+
+def test_sparse_pixels_preserved():
+    """Two isolated IR depths produce exactly two filled RGB pixels (identity reg)."""
+    reg = _identity_reg(W=64, H=48)
+    depth_ir = np.zeros((reg.ir_height, reg.ir_width), dtype=np.uint16)
+    depth_ir[0, 0] = 1000
+    depth_ir[reg.ir_height - 1, reg.ir_width - 1] = 1000
+    depth_rgb = align_depth_to_rgb(depth_ir, reg)
+    assert (depth_rgb > 0).sum() == 2
+
+
+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
+    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
+    reg = CameraRegistration(
+        K_rgb=K_rgb, K_ir=K_ir,
+        R_ir_to_rgb=np.eye(3, dtype=np.float64),
+        t_ir_to_rgb=np.zeros(3, dtype=np.float64),
+        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
+    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