fix: don't apply default plugs or layout to classic-confinement core24 snaps

[claude]

Requested via Slack thread

Before: building a core24 snap with confinement: classic still generated the default content-snap plugs (gnome-46-2404, gtk-3-themes, icon-themes, sound-themes, gpu-2404), the injected browser-support plug, app-level default plugs, and the default layout in snapcraft.yaml. The snap store review rejects classic snaps that declare plugs or layout, so classic core24 snaps could not pass review.

After: classic-confinement core24 snaps declare no plugs (root or app level) and no layout, restoring the guard the legacy builder has had since #2730. The core24 rewrite (#9517) lost this guard. Strict-confinement output is unchanged.

Details:

• Explicitly configured plugs/layout are still honored under classic, but a log.warn now notes the store review is likely to reject them.
• Because classic no longer gets the injected browser-support (allow-sandbox) plug, the generated launcher script now passes --no-sandbox and the unused setuid chrome-sandbox helper is stripped from the staged app — same runtime behavior the legacy builder applies to classic snaps.
• Plug/layout resolution in SnapCore24 was refactored from nested ternaries into explicit early-return helpers (resolvePlugs, resolveLayout, buildDefaultRootPlugs); behavior is otherwise identical.
• The snapcraftTest snapshot entry for the new classic test was written by hand; CI will correct it if it is off.

Fixes #10057

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Generated by Claude Code

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🦋 Changeset detected

Latest commit: c9cc82b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 8 packages

Name	Type
app-builder-lib	Patch
dmg-builder	Patch
electron-builder-squirrel-windows	Patch
electron-builder	Patch
electron-forge-maker-appimage	Patch
electron-forge-maker-nsis-web	Patch
electron-forge-maker-nsis	Patch
electron-forge-maker-snap	Patch

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