Add an opt-in scroll-to-zoom setting for SVG viewers

[rafaelha]

Summary

• Add the VS Code setting stim.svgZoomMode, available in user, workspace, and folder settings.
• Keep ctrlScroll as the default: Ctrl/Cmd+wheel zooms; plain scrolling pans or steps through single-layer slices.
• Offer scroll as an opt-in mode: plain scrolling zooms, and the existing left-button drag gesture pans. Slice layers remain accessible through the toolbar and keyboard.
• Apply configuration changes immediately to existing viewers without re-rendering the diagram or resetting zoom/pan.
• Include circuit and DEM SVG viewers; leave interactive 3D viewers unchanged.
• Document both modes and add seven viewer/settings regression tests to CI, without new dependencies.

Configuration

"stim.svgZoomMode": "scroll"

Set it back to ctrlScroll for the existing behavior.

Verification

• npx tsc --noEmit -p tsconfig.json
• npm run build
• npm run test:viewer (7 tests)
• npm run test:engine (28 tests)
• npm run test:grammar
• npm run test:wasm
• Local VSIX packaging with vsce
• Real-browser Playwright smoke checks at 1000x760 and 390x650: native scroll/pan, Ctrl+wheel zoom, live mode changes preserving the viewport, unmodified wheel zoom, and mouse-drag panning. Inspected screenshots of the actual WASM-rendered circuit.
• git diff --check

No version bump, release, or marketplace publication. No VSIX files are committed.