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/bus.rs

@@ -0,0 +1,58 @@
+//! NATS connection layer.
+//!
+//! Connection parameters follow the production-proven Vigilance profile:
+//! infinite reconnects with capped exponential backoff, 30s pings to detect
+//! zombie TCP in ~60s, and a deep client-side send queue so telemetry buffers
+//! through broker outages instead of erroring.
+
+use anyhow::{Context, Result};
+use std::time::Duration;
+
+use crate::config::Settings;
+
+pub async fn connect(cfg: &Settings) -> Result<async_nats::Client> {
+    let (url, force_tls) = normalize_url(&cfg.nats_url);
+
+    let mut opts = async_nats::ConnectOptions::new()
+        .name("corrosion-host-agent")
+        .retry_on_initial_connect()
+        .max_reconnects(None)
+        .ping_interval(Duration::from_secs(30))
+        .client_capacity(8192)
+        .reconnect_delay_callback(|attempts| {
+            Duration::from_millis(std::cmp::min(attempts as u64 * 100, 8_000))
+        })
+        .event_callback(|event| async move {
+            match event {
+                async_nats::Event::Disconnected => tracing::warn!("nats disconnected"),
+                async_nats::Event::Connected => tracing::info!("nats connected"),
+                other => tracing::debug!("nats event: {other}"),
+            }
+        });
+
+    if force_tls {
+        opts = opts.require_tls(true);
+    }
+    if let Some(token) = &cfg.nats_token {
+        opts = opts.token(token.clone());
+    }
+
+    let client = opts
+        .connect(&url)
+        .await
+        .with_context(|| format!("connecting to NATS at {url}"))?;
+
+    Ok(client)
+}
+
+/// Accept `tls://` / `nats+tls://` URL schemes by translating to `nats://` +
+/// an explicit TLS requirement.
+fn normalize_url(raw: &str) -> (String, bool) {
+    if let Some(rest) = raw.strip_prefix("tls://") {
+        (format!("nats://{rest}"), true)
+    } else if let Some(rest) = raw.strip_prefix("nats+tls://") {
+        (format!("nats://{rest}"), true)
+    } else {
+        (raw.to_string(), false)
+    }
+}