Fix Linux AppImage runtime mount lifecycle

[SawyerHood]

Human comments

This pull request replaces #1781 because the source branch for that pull request was deleted.

The replacement preserves the original work and gives co-author credit to @salemsayed.

What was wrong

The packaged desktop started the long-lived bb-app bridge through an executable inside the desktop AppImage mount. The bridge and its descendants could continue after that mount disappeared. These processes could then fail with SIGBUS or BUS_ADRERR errors.

What changed

• Start the packaged Linux runtime through the outer AppImage.
• Give the owned runtime an independent FUSE mount.
• Resolve the bridge path inside the new runtime mount.
• Keep the AppImage supervisor as the process-group leader until all descendants exit.
• Keep the original bridge path in the command line for stale-process verification.
• Add an AppImage lifecycle smoke test to the Linux build and release jobs.

This change does not alter the host daemon protocol.

How you verified

• pnpm exec turbo run typecheck test --filter=@bb/desktop
• The desktop typecheck passed.
• All 235 desktop tests passed.
• The prior Linux package smoke passed on Fix Linux AppImage runtime mount lifecycle #1781.
• git diff --check origin/main...HEAD

Replaces #1781.

AGENT GENERATED