fix: the single-color filter no longer takes line art for a blank image (#4483)

Five of Todding's Discord doodles (pencil lines on white, 3000px) were skipped
on import as "single color", so their posts showed text and no image. The
predicate sampled a 64px BILINEAR thumbnail, which blends thin strokes into
the paper, and 0.95 "one color" is below how white a doodle is.

- Sample 256x256 by NEAREST, so each sample is a real pixel.
- Default threshold 0.995: blank means essentially blank. Migration 0116 moves
  a stored 0.95 (the old default) with it; the settings slider now spans
  0.9-1 in 0.005 steps so the value is reachable.
- The Cleanup audit shares the predicate, so it stops flagging sketches too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_audits_single_color.py

@@ -40,3 +40,48 @@ def test_single_color_evaluate_handles_rgba_input():
     # Alpha channel should be ignored — only RGB matters for the rule.
     im = Image.new("RGBA", (50, 50), (100, 100, 100, 128))
     assert single_color.evaluate(im, threshold=0.9, tolerance=10) is True
+
+
+# -- line art is a drawing, not a blank (#4483) ---------------------------------
+
+_T = 0.995  # the default since alembic 0116
+_TOL = 30
+
+
+def _doodle(size=(1500, 2000), lines=12):
+    """Pencil line art on white: a few thin dark strokes plus faint
+    construction lines — a couple of percent ink, the rest paper."""
+    from PIL import ImageDraw
+
+    im = Image.new("RGB", size, (255, 255, 255))
+    draw = ImageDraw.Draw(im)
+    w, h = size
+    for i in range(lines):
+        x = (i + 1) * w // (lines + 1)
+        draw.line([(x, 0), (w - x, h)], fill=(70, 60, 60), width=3)
+        draw.line([(0, (i + 1) * h // (lines + 1)), (w, h - x)], fill=(235, 235, 235), width=2)
+    return im
+
+
+def test_line_art_on_white_is_not_single_color():
+    """Five of Todding's Discord doodles were skipped as blank: a smoothing
+    64px thumbnail blended the strokes into the paper."""
+    assert single_color.evaluate(_doodle(), threshold=_T, tolerance=_TOL) is False
+
+
+def test_a_blank_page_with_a_small_mark_is_still_single_color():
+    from PIL import ImageDraw
+
+    im = Image.new("RGB", (1000, 1000), (250, 250, 250))
+    ImageDraw.Draw(im).rectangle([480, 480, 520, 520], fill=(40, 40, 40))
+    assert single_color.evaluate(im, threshold=_T, tolerance=_TOL) is True
+
+
+def test_a_noisy_solid_fill_is_still_single_color():
+    """JPEG noise on a placeholder stays inside the tolerance."""
+    im = Image.new("RGB", (600, 600), (30, 90, 160))
+    for i in range(0, 600 * 600, 7):
+        x, y = i % 600, i // 600
+        d = (i % 21) - 10
+        im.putpixel((x, y), (30 + d, 90 + d, 160 + d))
+    assert single_color.evaluate(im, threshold=_T, tolerance=_TOL) is True