feat(host-agent): Rust rewrite Phase 0 — multi-instance foundation, v2 wire protocol, real telemetry

New corrosion-host-agent/ crate (Go companion-agent stays as behavior
reference until parity). Wire protocol v2 per COA-B: instance-scoped
subjects corrosion.{license}.{instance}.* + host-level .host.* — spec
in PROTOCOL.md, designed for the license->host->instance fleet model.

- Multi-instance TOML config in the foundation, not retrofitted
- NATS layer on the Vigilance production profile (infinite reconnect,
  capped backoff, 30s ping, 8192-msg offline buffer)
- Heartbeat with real sysinfo telemetry — Go agent shipped hardcoded
  disk/cpu placeholders; this is the panel's first true Resources data
- Connectivity prober (outbound TCP, periodic + on-demand)
- Host cmd channel (ping/probe/sysinfo), going-offline beacon,
  CancellationToken shutdown
- Live-fire verified against production NATS; artifacts: 3.7MB static
  linux-musl, 3.8MB windows .exe (static CRT)

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

corrosion-host-agent/src/main.rs

@@ -0,0 +1,168 @@
+//! Corrosion Host Agent — multi-game ops runtime.
+//!
+//! Phase 0: NATS connectivity, real host telemetry, multi-instance config,
+//! connectivity prober, host command channel. Process control, file ops, and
+//! game adapters arrive in Phase 1+ (see PROTOCOL.md).
+
+mod agent;
+mod bus;
+mod config;
+mod hostcmd;
+mod prober;
+mod subjects;
+mod telemetry;
+mod version;
+
+use anyhow::{Context, Result};
+use clap::{Parser, Subcommand};
+use std::path::PathBuf;
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+use tokio::sync::RwLock;
+use tokio_util::sync::CancellationToken;
+
+use crate::agent::Agent;
+
+#[derive(Parser)]
+#[command(name = "corrosion-host-agent", version = version::VERSION, about)]
+struct Cli {
+    /// Path to agent.toml (default: /etc/corrosion/agent.toml on Linux,
+    /// C:\ProgramData\Corrosion\agent.toml on Windows)
+    #[arg(long, short = 'c')]
+    config: Option<PathBuf>,
+
+    #[command(subcommand)]
+    command: Option<Command>,
+}
+
+#[derive(Subcommand)]
+enum Command {
+    /// Validate the config file and exit.
+    Check,
+    /// Print full version (semver, git hash, build timestamp) and exit.
+    Version,
+}
+
+fn main() -> Result<()> {
+    let cli = Cli::parse();
+    let config_path = cli.config.unwrap_or_else(config::default_config_path);
+
+    match cli.command {
+        Some(Command::Version) => {
+            println!("corrosion-host-agent {}", version::long());
+            Ok(())
+        }
+        Some(Command::Check) => {
+            let settings = config::load(&config_path)?;
+            println!(
+                "config ok: license {}, {} instance(s), nats {}",
+                settings.license_id,
+                settings.instances.len(),
+                settings.nats_url
+            );
+            Ok(())
+        }
+        None => {
+            let settings = config::load(&config_path)?;
+            init_logging(&settings.log_level);
+            tokio::runtime::Builder::new_multi_thread()
+                .enable_all()
+                .build()
+                .context("building tokio runtime")?
+                .block_on(run(settings))
+        }
+    }
+}
+
+fn init_logging(level: &str) {
+    let filter = tracing_subscriber::EnvFilter::try_from_default_env()
+        .unwrap_or_else(|_| tracing_subscriber::EnvFilter::new(level));
+    tracing_subscriber::fmt()
+        .with_env_filter(filter)
+        .with_target(false)
+        .init();
+}
+
+async fn run(settings: config::Settings) -> Result<()> {
+    tracing::info!(
+        "corrosion-host-agent {} starting: license {}, {} instance(s)",
+        version::long(),
+        settings.license_id,
+        settings.instances.len()
+    );
+    for inst in &settings.instances {
+        tracing::info!("  instance '{}' ({}) at {}", inst.id, inst.game, inst.root.display());
+    }
+
+    let nats = bus::connect(&settings).await?;
+
+    let agent = Arc::new(Agent {
+        cfg: settings,
+        nats,
+        started: Instant::now(),
+        last_probe: RwLock::new(None),
+        shutdown: CancellationToken::new(),
+    });
+
+    let mut handles = Vec::new();
+    handles.push(tokio::spawn(telemetry::run(agent.clone())));
+    handles.push(tokio::spawn(prober::run_loop(agent.clone())));
+    {
+        let agent = agent.clone();
+        handles.push(tokio::spawn(async move {
+            if let Err(e) = hostcmd::run(agent).await {
+                tracing::error!("host command handler failed: {e:#}");
+            }
+        }));
+    }
+
+    wait_for_shutdown_signal().await;
+    tracing::info!("shutdown signal received");
+    agent.shutdown.cancel();
+
+    // Best-effort offline beacon so the panel flips to offline immediately
+    // instead of waiting out the heartbeat staleness window.
+    let beacon = subjects::host_going_offline(&agent.cfg.license_id);
+    let _ = tokio::time::timeout(
+        Duration::from_millis(500),
+        agent.nats.publish(beacon, "{}".into()),
+    )
+    .await;
+
+    match tokio::time::timeout(
+        Duration::from_secs(10),
+        futures::future::join_all(handles),
+    )
+    .await
+    {
+        Ok(_) => tracing::info!("all subsystems stopped cleanly"),
+        Err(_) => tracing::warn!("shutdown timeout: some subsystems did not stop within 10s"),
+    }
+
+    let _ = agent.nats.flush().await;
+    tracing::info!("corrosion-host-agent stopped");
+    Ok(())
+}
+
+async fn wait_for_shutdown_signal() {
+    #[cfg(unix)]
+    {
+        use tokio::signal::unix::{signal, SignalKind};
+        let mut sigterm = match signal(SignalKind::terminate()) {
+            Ok(s) => s,
+            Err(e) => {
+                tracing::error!("SIGTERM handler failed: {e}; falling back to ctrl-c only");
+                let _ = tokio::signal::ctrl_c().await;
+                return;
+            }
+        };
+        tokio::select! {
+            _ = tokio::signal::ctrl_c() => {}
+            _ = sigterm.recv() => {}
+        }
+    }
+    #[cfg(not(unix))]
+    {
+        let _ = tokio::signal::ctrl_c().await;
+    }
+}