persistent logs + quieter init coordination

[ananthb]

Summary

Two changes bundled — both improve the pivot's post-mortem UX.

Persistent logs

--log-persist-path (default /var/log/xmorph) writes xmorph's own log and the entrypoint's stdout+stderr to a directory on the old rootfs (default). Optional --log-persist-device picks a specific pre-pivot mount to host it. Because the directory survives the kernel restart triggered by RebootOnFailure, you can tail it on the recovered OS to see why the pivoted install died — no more "box came back, no idea what happened."

• Pre-pivot check confirms the resolved directory is writable (mkdir + probe file) and aborts if not.
• Requires KeepOldRoot != \"\" (no persistent logs without an old root to persist to).
• Empty --log-persist-path disables.
• Slog is teed to <dir>/xmorph.log, the entrypoint's stdout+stderr to <dir>/entrypoint.log.

Quieter init coordination

initsys.Coordinator.TransitionToRescue now isolates basic.target instead of rescue.target on systemd hosts. rescue.target activates rescue.service which runs sulogin on the console — with a locked root, that looks like an emergency prompt during every pivot. basic.target stops all user services (same coverage) but keeps journal + dbus and doesn't touch tty state.

Test plan

• go test ./internal/... on Linux (endeavour): all pass.
• New TestSuperviseTeesToLogWriter confirms both stdout and stderr land in the writer.
• configwrite_test extended for LogPersistDir round-trip and empty-omission.
• Real hardware: pivot enterprise, deliberately fail install.sh, confirm the log appears on the recovered NixOS under /var/log/xmorph/.