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

[sLuCHaa]

Summary

Hatched content plotted incorrectly in the PDF / plot export — especially fills nested inside a block INSERT (a title-block logo, for example): they were missing, mis-spaced, or drew phantom bars. This PR makes the PDF/plot output match AutoCAD.

Fixes

1. Export block-internal hatch fills
The export path (paper_canvas_hatches) collected only hatches owned directly by the layout block, so a hatch nested in a block INSERT was dropped and the block printed as bare monochrome outlines. The insert-explosion the viewport already performs is extracted into a shared exploded_insert_hatch_models(), now called from both the viewport (synced_hatch_models) and the export — 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 × pattern_scale, ignoring the resolved line geometry the DWG stores on the hatch itself. When a drawing was authored against a different base spacing (e.g. imperial 0.125 vs the catalog's metric 3.175 for ANSI31), lines came out up to ~25× (inch→mm) 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 lookup 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 (k_lo > k_hi) and emitted nothing — silently dropping the fill. It now caps 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. Such paths are 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. The export now mirrors 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.

Tests

Adds tests/block_hatch_export.rs:

• block-internal hatch reaches the export set,
• stored-line spacing is honoured (not re-scaled from the catalog),
• far-from-origin fills are not dropped,
• TEXTBOX paths are not filled.

All self-contained (no external drawing required).

Scope

Export/plot path only. An unrelated on-screen GPU hatch-clipping nuance (dense pattern fills rendering over their bounding box in the viewport) is not part of this PR — it does not affect the plotted output.