perf(grid): virtualize grid rendering

[JustYannicc]

Summary

• Virtualize Raycast Grid rendering by flattening grouped sections into positioned virtual rows and rendering only the visible/overscan window.
• Preserve hidden item/action registration, selection, context actions, keyboard navigation, scrolling, accessories, and root/section layout options (columns, itemSize, aspectRatio, fit, inset).
• Add a focused Grid performance/regression script for visible-cell counts, selected-item scrolling, and section layout overrides.

Metrics

• Before runtime edits: node scripts/test-grid-virtualization.mjs reported baseline eager cells=5000, contentResolutions=5000, durationMs=0.589 for a 5,000-item grouped Grid.
• After virtualization: the same 5,000-item scenario reports virtualized visibleCells=30, renderedRows=7, totalRows=1127, with selection-scroll coverage for item 4242.
• Final focused run: baseline eager cells=5000, contentResolutions=5000, durationMs=0.342; virtualized visibleCells=30, totalCells=5000, renderedRows=7, totalRows=1127.

Checks

• node scripts/test-grid-virtualization.mjs passes.
• npm test passes (Renderer crash recovery (live Electron) skipped because Electron is not launchable here).
• npm run build:renderer passes.
• node scripts/check-i18n.mjs completes successfully; it still reports the existing Italian locale gaps for settings.ai.whisper.vocabulary and settings.extensions.appSearchScope.
• Codex LSP diagnostics are clean for the changed Grid files.
• ./node_modules/.bin/tsc --noEmit --pretty false 2>&1 | rg "grid-runtime" || true reports no Grid-related typecheck errors. The full renderer typecheck remains red on unrelated existing files.

Risks / Notes

• The registration tree still mounts hidden Grid.Item elements so extension APIs/actions keep working; the expensive visual cell rendering and icon resolution are limited to visible rows.
• Live Electron interaction was not available in this environment, so scrolling behavior is covered by the pure layout regression rather than an end-to-end window test.

[JustYannicc]

Superseded by the consolidated renderer performance PR #533: #533. Closing this scoped source PR so review stays centralized there.