fix: compute seam positions from actual tile grid

Replicate SplitImageToTileList's get_grid_coords logic to find real
overlap regions between adjacent tiles. Fixes three bugs:

1. Bands were at overlap start instead of center (off by overlap/2)
2. Spurious bands generated beyond the actual tile grid
3. Edge tile seams placed at wrong position (ignoring boundary shift)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_seam_mask.py

@@ -15,16 +15,20 @@ def test_output_shape():
 
 
 def test_seam_positions():
+    """Seam bands should be centered at the overlap midpoint between adjacent tiles."""
     node = GenerateSeamMask()
     result = node.generate(image_width=2048, image_height=2048,
                            tile_width=1024, tile_height=1024,
                            overlap=128, seam_width=64)
     mask = result[0]
-    # Stride = 1024 - 128 = 896
-    # Seams at x=896, 1792 and y=896, 1792
-    assert mask[0, 0, 896, 0].item() == 1.0, "Center of vertical seam should be white"
-    assert mask[0, 896, 0, 0].item() == 1.0, "Center of horizontal seam should be white"
+    # Tiles: [0,1024), [896,1920), [1024,2048)
+    # Overlap between tile 0 and 1: [896, 1024), center=960
+    # Band should be at [928, 992)
+    assert mask[0, 0, 960, 0].item() == 1.0, "Center of overlap (960) should be white"
+    assert mask[0, 960, 0, 0].item() == 1.0, "Horizontal seam center should be white"
     assert mask[0, 0, 400, 0].item() == 0.0, "Far from any seam should be black"
+    # Old wrong position (stride=896) should NOT be in the band
+    assert mask[0, 0, 896, 0].item() == 0.0, "Start of overlap (896) should be outside the band"
 
 
 def test_no_seams_single_tile():
@@ -37,17 +41,50 @@ def test_no_seams_single_tile():
     assert mask.sum().item() == 0.0, "Single tile image should have no seams"
 
 
-def test_seam_band_width():
+def test_seam_band_width_no_overlap():
+    """With overlap=0, seam center is at tile boundary."""
     node = GenerateSeamMask()
     result = node.generate(image_width=2048, image_height=1024,
                            tile_width=1024, tile_height=1024,
                            overlap=0, seam_width=64)
     mask = result[0]
-    # Stride = 1024, seam at x=1024, band from 992 to 1056
-    assert mask[0, 0, 1023, 0].item() == 1.0, "Inside band should be white"
+    # Tiles: [0,1024), [1024,2048). Overlap: [1024,1024) = empty.
+    # Center at 1024, band [992, 1056)
+    assert mask[0, 0, 1024, 0].item() == 1.0, "Seam center should be white"
     assert mask[0, 0, 991, 0].item() == 0.0, "Outside band should be black"
 
 
+def test_no_spurious_bands():
+    """Should not generate bands beyond the actual tile grid."""
+    node = GenerateSeamMask()
+    # 2816px with 1024 tiles, stride=896: 3 tiles, 2 seams
+    result = node.generate(image_width=2816, image_height=1024,
+                           tile_width=1024, tile_height=1024,
+                           overlap=128, seam_width=64)
+    mask = result[0]
+    # Tiles: [0,1024), [896,1920), [1792,2816) — 3 tiles, 2 vertical seams
+    # Seam 0-1: overlap [896,1024), center=960
+    # Seam 1-2: overlap [1792,1920), center=1856
+    assert mask[0, 0, 960, 0].item() == 1.0, "Seam 0-1 center should be white"
+    assert mask[0, 0, 1856, 0].item() == 1.0, "Seam 1-2 center should be white"
+    # x=2688 was a spurious band in the old code — should be black now
+    assert mask[0, 0, 2688, 0].item() == 0.0, "No spurious band beyond tile grid"
+
+
+def test_edge_tile_seam_position():
+    """Edge tile seam should be at the actual overlap center, not at n*stride."""
+    node = GenerateSeamMask()
+    # 2048px: tiles [0,1024), [896,1920), [1024,2048)
+    # Edge seam between tile 1 and 2: overlap [1024,1920), center=1472
+    result = node.generate(image_width=2048, image_height=1024,
+                           tile_width=1024, tile_height=1024,
+                           overlap=128, seam_width=64)
+    mask = result[0]
+    assert mask[0, 0, 1472, 0].item() == 1.0, "Edge tile seam center (1472) should be white"
+    # Old wrong position
+    assert mask[0, 0, 1792, 0].item() == 0.0, "Old position (1792) should be black"
+
+
 def test_values_are_binary():
     node = GenerateSeamMask()
     result = node.generate(image_width=2048, image_height=2048,
@@ -62,6 +99,8 @@ if __name__ == "__main__":
     test_output_shape()
     test_seam_positions()
     test_no_seams_single_tile()
-    test_seam_band_width()
+    test_seam_band_width_no_overlap()
+    test_no_spurious_bands()
+    test_edge_tile_seam_position()
     test_values_are_binary()
     print("All tests passed!")