fix(preview): capture screenshots from hidden tabs

[incognitojam]

Summary

Background preview tabs remain parked far outside the Electron compositor. Their DOM automation stays available, but WebContents.capturePage() can reject with UnknownVizError or never settle because the guest has no captureable surface.

This change gives snapshot requests a short-lived, reference-counted compositor lease. Only the target webview moves in-window with opacity: 0, the host waits for guest render frames, and the lease is always released after capture. Native snapshot capture is also bounded and single-flight so a stuck promise cannot start overlapping compositor copies.

This is the screenshot portion of #170. Recording and preview visibility semantics remain separate follow-ups.

Testing

• vp test run apps/desktop/src/preview/Manager.test.ts apps/web/src/browser/browserSurfaceStore.test.ts apps/web/src/browser/hostedBrowserWebviewStyle.test.ts
• vp run --filter @t3tools/web typecheck
• vp run --filter @t3tools/desktop typecheck
• Targeted lint for all changed files
• Electron 41.5.0 compositor probe: five parked-to-leased captures returned non-empty 1600x1200 images in 5-76 ms
• Dev desktop MCP verification: visible capture succeeded; two fresh open: false tabs stayed visible: false before and after valid 1280x800 PNG snapshots; fresh open: true, show: true background tabs also captured successfully

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Written by an agent (T3 Code, gpt-5.6-sol).