Fast-exiting processes (e.g. echo) sent data/end events before
start() subscribed to the broadcast channels, causing the stream
to hang indefinitely and the exec RPC to time out with 502.
Move channel subscription into spawn_process, before reader/waiter
threads start, and return pre-subscribed receivers via SpawnedProcess.
Three bugs fixed:
1. PTY connections failed because home directory was hardcoded as
/home/{username} instead of reading from /etc/passwd. For root,
this produced /home/root/ which doesn't exist — CWD validation
rejected every PTY Start request without explicit cwd. Fixed all
6 locations to use user.dir from nix::unistd::User.
2. MMDS polling silently failed to parse metadata because the
logs_collector_address field lacked #[serde(default)]. The host
agent only sends instanceID + envID — missing "address" field
caused every deserialize attempt to fail, so .WRENN_SANDBOX_ID
and .WRENN_TEMPLATE_ID were never written. Also added error
logging and create_dir_all before file writes.
3. Metrics CPU values were non-deterministic because a fresh
sysinfo::System was created per request with a 100ms sleep
between reads. Replaced with a background thread that samples
CPU at fixed 1-second intervals via a persistent System instance,
matching gopsutil's internal caching behavior. Metrics endpoint
now reads cached atomic values — no blocking, consistent window.
Also: close master PTY fd in child pre_exec, add process.Start
request logging, bump version to 0.2.0.
