Fixes LXC bind mount issue where socket-level mounts break when the
socket is recreated by systemd. Following Codex's recommendation to
bind mount the directory instead of the file.
Changes:
- Socket path: /run/pulse-temp-proxy/pulse-temp-proxy.sock
- Systemd: RuntimeDirectory=pulse-temp-proxy (auto-creates /run/pulse-temp-proxy)
- Systemd: RuntimeDirectoryMode=0770 for group access
- LXC mount: Bind entire /run/pulse-temp-proxy directory
- Install script: Upgrades old socket-level mounts to directory-level
- Install script: Detects and handles bind mount changes
This survives socket recreations and container restarts. The directory
mount persists even when systemd unlinks/recreates the socket file.
Related to #528
c7bb76c12e