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/scene/model/hatch_model.rs

@@ -210,8 +210,15 @@ impl HatchModel {
             if k_lo > k_hi {
                 std::mem::swap(&mut k_lo, &mut k_hi);
             }
-            k_lo = k_lo.max(-MAX_LINES_PER_FAMILY);
-            k_hi = k_hi.min(MAX_LINES_PER_FAMILY);
+            // Cap the line COUNT (span), not the absolute index. A hatch far
+            // from the pattern origin (0,0) — e.g. a fine-spaced fill at large
+            // drawing coordinates — has large-magnitude k at both ends but a
+            // small span. Clamping the absolute index to ±MAX_LINES_PER_FAMILY
+            // would invert the range (k_lo > k_hi) and emit nothing, silently
+            // dropping the whole fill.
+            if k_hi.saturating_sub(k_lo) > MAX_LINES_PER_FAMILY {
+                k_hi = k_lo.saturating_add(MAX_LINES_PER_FAMILY);
+            }
 
             let period: f32 = family.dashes.iter().map(|d| d.abs()).sum::<f32>() * scale;
             let has_dashes = !family.dashes.is_empty() && period > 1e-6;