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/telemetry.rs

@@ -0,0 +1,175 @@
+//! Host heartbeat: real telemetry, never fabricated.
+//!
+//! The Go agent shipped `disk_free_mb: 50000` and `cpu_percent: 0.0` as
+//! hardcoded placeholders. This module is the first time the panel's
+//! Resources view receives the truth. Anything we cannot measure is omitted
+//! or null — never invented.
+
+use chrono::{SecondsFormat, Utc};
+use rand::Rng;
+use serde::Serialize;
+use std::path::Path;
+use std::sync::Arc;
+use std::time::Duration;
+use sysinfo::{Disks, System};
+
+use crate::agent::Agent;
+use crate::prober::ProbeReport;
+use crate::subjects;
+use crate::version;
+
+#[derive(Debug, Serialize)]
+pub struct HeartbeatPayload {
+    /// Wire schema version — lets the backend distinguish v2 host heartbeats
+    /// from legacy Go companion heartbeats during any transition window.
+    pub schema: u32,
+    pub timestamp: String,
+    pub agent: AgentInfo,
+    pub host: HostInfo,
+    pub instances: Vec<InstanceInfo>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub probe: Option<ProbeReport>,
+}
+
+#[derive(Debug, Serialize)]
+pub struct AgentInfo {
+    pub version: String,
+    pub commit: String,
+    pub os: String,
+    pub arch: String,
+    pub uptime_seconds: u64,
+}
+
+#[derive(Debug, Serialize)]
+pub struct HostInfo {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub hostname: Option<String>,
+    pub cpu_percent: f32,
+    pub cpu_cores: usize,
+    pub mem_total_mb: u64,
+    pub mem_used_mb: u64,
+    pub uptime_seconds: u64,
+    pub disks: Vec<DiskInfo>,
+}
+
+#[derive(Debug, Serialize)]
+pub struct DiskInfo {
+    pub mount: String,
+    pub total_mb: u64,
+    pub free_mb: u64,
+}
+
+#[derive(Debug, Serialize)]
+pub struct InstanceInfo {
+    pub id: String,
+    pub game: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub label: Option<String>,
+    /// Phase 0 states: `configured` (root exists) or `missing_root`.
+    /// Phase 1 adds live process states (running/stopped/crashed).
+    pub state: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub root_disk_free_mb: Option<u64>,
+}
+
+pub async fn run(agent: Arc<Agent>) {
+    let cancel = agent.shutdown.clone();
+    let mut sys = System::new();
+
+    // CPU usage is a delta between refreshes; prime it once so the first
+    // heartbeat carries a real figure instead of 0.
+    sys.refresh_cpu_usage();
+    tokio::time::sleep(Duration::from_millis(250)).await;
+
+    loop {
+        let payload = collect(&agent, &mut sys).await;
+        match serde_json::to_vec(&payload) {
+            Ok(bytes) => {
+                let subject = subjects::host_heartbeat(&agent.cfg.license_id);
+                if let Err(e) = agent.nats.publish(subject, bytes.into()).await {
+                    tracing::warn!("heartbeat publish failed: {e}");
+                } else {
+                    tracing::debug!(
+                        "heartbeat sent: cpu {:.1}%, {} instance(s)",
+                        payload.host.cpu_percent,
+                        payload.instances.len()
+                    );
+                }
+            }
+            Err(e) => tracing::error!("heartbeat serialize failed: {e}"),
+        }
+
+        let jitter = rand::thread_rng().gen_range(0.8..1.2);
+        let interval = Duration::from_secs_f64(agent.cfg.heartbeat_seconds as f64 * jitter);
+        tokio::select! {
+            _ = tokio::time::sleep(interval) => {}
+            _ = cancel.cancelled() => {
+                tracing::info!("telemetry stopping");
+                break;
+            }
+        }
+    }
+}
+
+pub async fn collect(agent: &Agent, sys: &mut System) -> HeartbeatPayload {
+    sys.refresh_cpu_usage();
+    sys.refresh_memory();
+    let disks = Disks::new_with_refreshed_list();
+
+    let disk_infos: Vec<DiskInfo> = disks
+        .iter()
+        .map(|d| DiskInfo {
+            mount: d.mount_point().to_string_lossy().to_string(),
+            total_mb: d.total_space() / 1_048_576,
+            free_mb: d.available_space() / 1_048_576,
+        })
+        .collect();
+
+    let instances = agent
+        .cfg
+        .instances
+        .iter()
+        .map(|inst| {
+            let exists = inst.root.exists();
+            InstanceInfo {
+                id: inst.id.clone(),
+                game: inst.game.clone(),
+                label: inst.label.clone(),
+                state: if exists { "configured" } else { "missing_root" }.to_string(),
+                root_disk_free_mb: disk_free_for_path(&disks, &inst.root),
+            }
+        })
+        .collect();
+
+    HeartbeatPayload {
+        schema: 2,
+        timestamp: Utc::now().to_rfc3339_opts(SecondsFormat::Secs, true),
+        agent: AgentInfo {
+            version: version::VERSION.to_string(),
+            commit: version::GIT_HASH.to_string(),
+            os: std::env::consts::OS.to_string(),
+            arch: std::env::consts::ARCH.to_string(),
+            uptime_seconds: agent.started.elapsed().as_secs(),
+        },
+        host: HostInfo {
+            hostname: System::host_name(),
+            cpu_percent: sys.global_cpu_usage(),
+            cpu_cores: sys.cpus().len(),
+            mem_total_mb: sys.total_memory() / 1_048_576,
+            mem_used_mb: sys.used_memory() / 1_048_576,
+            uptime_seconds: System::uptime(),
+            disks: disk_infos,
+        },
+        instances,
+        probe: agent.last_probe.read().await.clone(),
+    }
+}
+
+/// Free space on the disk whose mount point is the longest prefix of `path`.
+fn disk_free_for_path(disks: &Disks, path: &Path) -> Option<u64> {
+    disks
+        .iter()
+        .filter(|d| path.starts_with(d.mount_point()))
+        .max_by_key(|d| d.mount_point().as_os_str().len())
+        .map(|d| d.available_space() / 1_048_576)
+}