Bubblewrap parent-death signals follow the creating thread. The exact
31535e3 Linux RC killed both pipe and PTY background jobs when Tokio
retired that blocking thread, even though Engine stayed alive.
Spawn Linux bubblewrap background children on one process-lifetime thread.
Both process launch sites and both PTY consumers use the shared ExecEnv
path. Preserve --die-with-parent, namespace/mount confinement, filtered
environments, controlling-terminal setup, and existing stop/cleanup.
Verification: 6 existing focused macOS tests passed; formatting and diff
checks passed. Linux ARM64 diagnostic build passed with the existing
keyring/vendored feature (the initial default macOS cross-build lacked a
D-Bus sysroot). That binary passed 35 immediate terminal/confinement/Stop
observations and 11 reconnect observations after 53.7 seconds idle. Both
pipe and PTY jobs survived the identical 28-second failure control. Killing
Engine removed all four tracked sandbox processes and systemd recovered.
Source stayed frozen across builds and observations. Final native RC and
release qualification remain separate; no new tests or comments added.
Refs CW-6606, CW-7462.