feat: spotlight magnifier

[devmobasa]

Summary

• Add per-Spotlight magnification from 1× to 4×, with a configurable default and session persistence.
• Render magnified loupes from completed canvas/background pixels across solid boards, Freeze, Zoom, captures, thumbnails, and exports.
• Add three controls: the style pill, scrolling over a loupe, and an on-canvas drag handle.
• Preserve correct undo/history behavior across wheel gestures, autosave, shutdown, page and board changes, session replacement, and output transitions.

Interaction details

• Magnification changes in 0.25× steps.
• High-resolution wheels accumulate value120 units without prematurely applying partial ticks.
• Touchpad gestures remain one undo action until axis_stop; wheel-like sources use a quiet-period fallback.
• Locked topmost loupes claim scrolling without changing themselves or an overlapping loupe underneath.
• The on-canvas control remains reachable on panned, zoomed, and edge-positioned canvases.

Rendering and compatibility

• Existing sessions default older Spotlight shapes to 1×.
• Magnification is stored per shape, while the configured value seeds newly drawn Spotlights.
• Live transparent boards retain the normal opening and suggest Freeze when magnification lacks a complete pixel source.
• Transparent exports fail explicitly instead of silently producing an unmagnified result.
• PDF pages containing magnified Spotlights are rasterized for correct pixel sampling while page labels remain vector content.
• Adds Spotlight-specific rendering metrics and retained-surface lifecycle hardening.