feat(host-agent): Phase 1a process supervision — instance start/stop/restart/status + push state events

Per-instance ProcessSupervisor: tokio child spawn with proper arg list
(fixes Go's naive space-splitting), graceful SIGTERM with 30s budget
then force kill, monitor task classifying ordered-stop vs crash (exit
code captured), watch-channel state observable everywhere. Instance cmd
channel live on corrosion.{license}.{instance}.cmd (start/stop/restart/
status) with state events pushed on {instance}.status (keep-latest
semantics, documented). Heartbeats now carry live process state +
uptime per instance. Crate restructured lib+bin for integration tests.

Verified: 5 integration tests with real OS processes (lifecycle, crash
exit-code, restart recovery, unmanaged rejection, clean spawn failure)
+ live-NATS contract test (request-reply roundtrips, double-start
rejection, push events, heartbeat state) — all green.

Known limitation (documented): no PID adoption yet — agent restart
orphans a running game process to 'stopped' until panel restart.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

corrosion-host-agent/src/subjects.rs

@@ -17,14 +17,12 @@ pub fn host_going_offline(license: &str) -> String {
     format!("corrosion.{license}.host.going_offline")
 }
 
-/// Phase 1: per-instance command channel (start/stop/restart/rcon/...).
-#[allow(dead_code)]
+/// Per-instance command channel (start/stop/restart/status; rcon et al. to come).
 pub fn instance_cmd(license: &str, instance: &str) -> String {
     format!("corrosion.{license}.{instance}.cmd")
 }
 
-/// Phase 1: per-instance state-change events.
-#[allow(dead_code)]
+/// Per-instance state-change events.
 pub fn instance_status(license: &str, instance: &str) -> String {
     format!("corrosion.{license}.{instance}.status")
 }