PDF follow-ups: import layers, arcs and joins, names, background, page placement, snaps

- Unlayered raster images go with the PDF layer drawn last before them.
- Circular Bezier arcs import as arcs: bulged polyline segments with Join,
  ARC entities without it; straight chains still join.
- Imported PNGs go to "PDF Images" next to the PDF; import blocks are
  numbered imp31, imp32, ...
- Background darkness is its HSL lightness below one half.
- All chosen pages attach at the same insertion point.
- PDF geometry snaps as separate pieces: centre from the rim, intersections
  and perpendiculars.
- Automation: snap query with an optional base point.

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

src/scene/model/pdf_raster.rs

@@ -252,8 +252,6 @@ pub fn adjusted_pixels(path: &str, page: &str, raster: &PdfPage, adjust: PageAdj
             // (plus 20/255, capped at full), hue and saturation kept: black
             // becomes white, dark red light pink. On a light background the
             // colours stay as drawn.
-            // ponytail: offset and the light-background rule measured on the
-            // default backgrounds only (pure white on paper not measured).
             let (h, s, l) = rgb_to_hsl(rgb);
             rgb = hsl_to_rgb(h, s, (1.0 - l + 20.0 / 255.0).min(1.0));
         }