Bonsai: preview drawing SVG in camera viewport - #9472

Summary

This is a focused first slice for #6422. It adds an opt-in Generated SVG Overlay in the Drawings panel that reads the final combined drawing SVG and previews its straight-line publication styling directly in the active orthographic drawing camera viewport.

The overlay:

• reuses the already-generated final SVG; it does not add a second renderer or change the SVG generation pipeline;
• parses a bounded SVG/CSS subset for straight paths, lines, polylines/polygons, class/tag selectors, stroke color/width/opacity, and dash arrays;
• maps paper SVG coordinates back to the drawing camera plane and draws through Blender's POST_VIEW GPU path;
• caches parsed geometry and refreshes after Create Drawing;
• is opt-in, camera-only, and degrades safely when the final SVG is missing or contains unsupported features.

This PR deliberately does not attempt the complete publication preview yet. Fills, text, curves, transforms, clipping, markers/patterns, and paint servers remain outside this first bounded slice. In particular, cut/fill-color fidelity still needs a follow-up slice if this direction is accepted.

Testing

• Focused SVG parser/mapping tests: 17 passed + 11 subtests passed, including regression coverage for opacity="inherit".
• Python compile checks passed for all 8 touched/new Python files.
• Isolated Blender 4.5.4 handler/camera-gating/cache/refresh smoke checks passed.
• Black, Ruff, and git diff --check passed.
• Full Bonsai suite could not be collected locally because pytest-blender is unavailable.

Visual evidence

A local Windows visual harness used Blender 4.5.4 LTS with Bonsai 0.8.5-post1 and minimal integration adaptations for that older Bonsai API. The overlay/parser logic from this PR was used against a Bonsai-generated PLAN_VIEW.svg. These screenshots validate the stroke-overlay slice only, not fills/text/patterns or full print fidelity.

• Overlay OFF
• Overlay ON

AI assistance

OpenAI Codex was used to investigate, implement, test, and review this change. I reviewed the resulting patch and take responsibility for the contribution. New AI-generated files contain the repository-required disclosure.

Refs #6422