id:361232 svg generation issue - layers, text viewbox, mirrored ARC/ELLIPSE positions

[NicholasPhillips]

No description provided.

[Copilot]

Pull request overview

This PR significantly extends programs/dwg2SVG.c to address layering issues in the generated SVG output. The most substantive change is a new "block clone" mechanism that emits one <defs> <symbol> per (block_handle, effective_layer) combination so AutoCAD's "layer 0 inside a block inherits the INSERT's layer" rule is honoured. Several rendering improvements (ARC/ELLIPSE for mirrored extrusions, full SPLINE support via de Boor / fit-point sampling, MTEXT/ATTRIB bbox estimation, per-block origin shift to keep path coordinates in a precision-safe range) ride along.

Changes:

• Add BlockCombo table, collect_block_combos*, insert_effective_layer, effective_layer_name, and layer-0 inheritance plumbing; emit per-combo <symbol-{handle}-{eff_layer}> clones, and refactor output_BLOCK_HEADER into INSERT-emit (Pass 2a) + per-layer entity/ATTRIB-emit (Pass 2b) passes.
• Add output_SPLINE (de Boor evaluation, fit-point fallback), rewrite output_ARC / output_ELLIPSE to handle mirrored extrusions and partial elliptical arcs, and add MTEXT/ATTRIB/TEXT bbox estimation via extents_add_text_bbox / extents_add_rotated_bbox.
• Introduce current_block_offset_x/y (subtracted by transform_X/Y inside block defs) plus matching R·S·offset compensation in the <use> translate so large-coordinate blocks remain precision-safe; split output_INSERT_attribs out so attached ATTRIBs are emitted by the layer-aware caller rather than inline.

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