fix(cmux-next): keep hidden WebViews out of scene captures

[azooz2003-bit]

The v16 scene captures composited a parked Home WebView over the active Agent pane because the native base hid only selected pages. Composer and hint captures also froze a picker during its fade, and the sidebar scene restored width without selecting the tray look.

This follow-up to #17361 now hides every WKWebView while drawing the native base, snapshots only visible views with effective alpha, uses precomputed window coordinates in view order, waits for menu animations to finish, and selects the real tray look for sidebar-tiles. The render script fails closed if a manifest entry reports a hidden WebView composited.

Validation:

• git diff --check
• bash -n scripts/cmux-next/render-scenes.sh
• exact-head fleet build and real seven-scene capture will be attached before merge

Follow-up to #17361.

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Summary by cubic

Keeps hidden WebViews, parked tabs, and mid-animation menus out of scene captures so screenshots match what users see.

• Hides every WKWebView while drawing the native base instead of only selected pages, so a parked Home WebView no longer overlays the active Agent pane.
• Composits back only views visible at both selection and draw time, using precomputed window coordinates in view order and effective alpha across ancestors.
• Fails the capture if a hidden WebView would have been composited or a visible WebView fails to snapshot, and render-scenes.sh exits on those errors.
• Awaits menu animations with the Web Animations API and closes leftover menus before snapshotting.
• Resets the sidebar look per scene so only sidebar-tiles uses the tray look.

^{Written for commit d45d303. Summary will update on new commits.}

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Migrated from #17370 after correcting the PR author identity. The original head commit d45d303 is preserved.

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