fix(plot): correct hatch rendering in PDF / plot export

Hatched content — especially fills nested inside a block INSERT, such as a
title-block logo — plotted incorrectly: missing, mis-spaced, or with phantom
bars. Five related fixes make the PDF/plot output match AutoCAD.

1. Export block-internal hatch fills.
   The export collected only hatches owned directly by the layout block, so a
   hatch nested in a block INSERT was dropped and printed as bare monochrome
   outlines. The insert-explosion the viewport already does is extracted into a
   shared `exploded_insert_hatch_models()` and called from both the viewport
   (`synced_hatch_models`) and the export (`paper_canvas_hatches`), so a plot
   draws block-internal hatches identically to the screen.

2. Honour the hatch's own stored pattern-line spacing.
   `hatch_model_from_dxf` re-derived pattern spacing from the name-matched
   catalog entry x pattern_scale, ignoring the resolved line geometry the DWG
   stores on the hatch. When a drawing was authored against a different base
   spacing (imperial 0.125 vs the catalog's metric 3.175 for ANSI31), lines
   came out up to ~25x too coarse and a dense fill collapsed to a few stray
   lines. When the hatch carries its own line geometry it is now used directly
   (identity scale/angle); the catalog path remains the fallback.

3. Fill far-from-origin pattern hatches.
   `pattern_segments` clamped the ABSOLUTE scan-line index to
   +/-MAX_LINES_PER_FAMILY. A fine-spaced hatch far from the pattern origin has
   large-magnitude indices at both ends but a small span, so the clamp inverted
   the range and emitted nothing — silently dropping the fill. Cap the line
   count (span) instead of the absolute index.

4. Skip TEXTBOX boundary paths.
   TEXTBOX boundary paths (flag bit 3) are text bounding-boxes AutoCAD derives
   for island detection; they are never drawn or filled. Treating one as a fill
   boundary painted its rectangle solid — a phantom bar. It is now skipped when
   building the fill boundary.

5. Print white/ACI-7 hatch fills black on paper.
   Hatch fills arrive adapted to the dark screen background, so a white/ACI-7
   fill would vanish white-on-white on the sheet. Mirror the wire pass:
   near-white/near-yellow -> black, near-cyan -> dark blue, matching AutoCAD's
   colour-7-on-white plotting. Wipeouts keep their paper-white mask.

Adds `tests/block_hatch_export.rs` covering: block-internal hatch reaches the
export set, stored-line spacing is honoured, far-from-origin fills are not
dropped, and TEXTBOX paths are not filled.

src/io/pdf_export.rs

@@ -381,10 +381,29 @@ fn emit_hatch(ops: &mut Vec<Op>, hatch: &HatchModel, ox: f32, oy: f32) {
     if hatch.boundary.is_empty() {
         return;
     }
-    let [r, g, b, a] = hatch.color;
+    let [mut r, mut g, mut b, a] = hatch.color;
     if a < 0.01 {
         return;
     }
+    // Adapt hatch fills to the white sheet, mirroring the wire pass: colours
+    // arrive adapted to the (dark) screen background, so a white/ACI-7 fill
+    // would vanish white-on-white on paper. Force near-white/near-yellow → black
+    // and near-cyan → dark blue, matching AutoCAD's colour-7-on-white plotting.
+    // Genuine colours are untouched; WIPEOUTS keep their paper-white mask.
+    if hatch.name != "WIPEOUT_FILL" {
+        let is_light = r > 0.80 && g > 0.80 && b > 0.80;
+        let is_yellow = r > 0.80 && g > 0.70 && b < 0.30;
+        let is_cyan = r < 0.30 && g > 0.70 && b > 0.70;
+        if is_light || is_yellow {
+            r = 0.0;
+            g = 0.0;
+            b = 0.0;
+        } else if is_cyan {
+            r = 0.0;
+            g = 0.15;
+            b = 0.50;
+        }
+    }
     let world_ox = hatch.world_origin[0] as f32;
     let world_oy = hatch.world_origin[1] as f32;