GUI: embedding API for host apps, PNG-default preview

[BenHuddart]

Summary

• Adds gleplot.gui.open_editor(path, *, parent, settings, gle_executable) so host applications (e.g. Asymmetry) can embed the editor inside their own QApplication without touching app-global identity (name/organization/icon) or running an event loop.
• Adds MainWindow.open_path() and MainWindow.apply_gle_executable() public wrappers for embedders, and lets gleplot-gui accept an optional .gle file argument on the CLI.
• Switches the live preview to PNG by default (more robust than GLE's Cairo SVG backend); SVG vector preview becomes an explicit opt-in via View ▸ Vector preview (SVG), with the one-time probe compile deferred until first opt-in instead of running at every startup.
• Documents a QtSvg limitation: GLE's Cairo backend implements box/legend occlusion via multi-subpath <clipPath> clipping, which QtSvg doesn't fully support, causing occluded content to render incorrectly in the SVG preview (the SVG file itself is correct in browsers). The SVG toggle now shows a tooltip and status-bar caveat steering users back to PNG when rendering looks wrong.
• Refreshes checked-in example .gle outputs (errorbar/dual-axis/secondary-y-axis examples) to match current generation.

Why

Host apps embedding gleplot's editor need a way to open it inside their own Qt application lifecycle rather than gleplot's GUI owning QApplication identity and the event loop. Separately, the SVG preview path was found to mis-render occluded content due to QtSvg's clip-path limitations, so PNG needed to become the safer default.

Test plan

• pytest tests/gui/test_app.py -v
• pytest tests/gui/test_svg_preview.py -v
• Manual: embed open_editor() in a host QApplication and confirm it opens without clobbering app identity
• Manual: toggle View ▸ Vector preview (SVG) and confirm the caveat/tooltip appears

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