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/config.rs

@@ -0,0 +1,186 @@
+//! Agent configuration: TOML file + environment overrides.
+//!
+//! Multi-instance is foundational, not bolted on: one agent supervises N game
+//! instances on the host, each declared as an `[[instance]]` block. Connection
+//! secrets may come from env so the config file can be world-readable-ish
+//! while the token is not.
+
+use anyhow::{bail, Context, Result};
+use serde::Deserialize;
+use std::collections::HashSet;
+use std::path::{Path, PathBuf};
+
+/// Instance ids share the NATS subject namespace with host-level segments.
+const RESERVED_INSTANCE_IDS: &[&str] = &["host", "cmd", "files", "update", "agent"];
+
+pub const SUPPORTED_GAMES: &[&str] = &["rust", "conan", "soulmask", "dune"];
+
+#[derive(Debug, Clone, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ConfigFile {
+    pub agent: AgentSection,
+    #[serde(default, rename = "instance")]
+    pub instances: Vec<InstanceConfig>,
+    #[serde(default)]
+    pub prober: ProberSection,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct AgentSection {
+    pub license_id: Option<String>,
+    pub nats_url: Option<String>,
+    pub nats_token: Option<String>,
+    #[serde(default = "default_heartbeat_seconds")]
+    pub heartbeat_seconds: u64,
+    #[serde(default = "default_log_level")]
+    pub log_level: String,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct InstanceConfig {
+    /// Short slug, unique per license: becomes a NATS subject segment.
+    pub id: String,
+    /// One of SUPPORTED_GAMES.
+    pub game: String,
+    /// Install root for this instance on the host.
+    pub root: PathBuf,
+    /// Optional human label shown in the panel.
+    #[serde(default)]
+    pub label: Option<String>,
+}
+
+#[derive(Debug, Clone, Default, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ProberSection {
+    #[serde(default = "default_probe_interval")]
+    pub interval_seconds: u64,
+    /// Extra TCP targets beyond the built-in defaults.
+    #[serde(default, rename = "target")]
+    pub targets: Vec<ProbeTargetConfig>,
+}
+
+#[derive(Debug, Clone, Deserialize)]
+#[serde(deny_unknown_fields)]
+pub struct ProbeTargetConfig {
+    pub name: String,
+    pub host: String,
+    pub port: u16,
+}
+
+fn default_heartbeat_seconds() -> u64 {
+    60
+}
+
+fn default_probe_interval() -> u64 {
+    300
+}
+
+fn default_log_level() -> String {
+    "info".to_string()
+}
+
+/// Fully-resolved settings after merging file + env. Everything required is
+/// present and validated.
+#[derive(Debug, Clone)]
+pub struct Settings {
+    pub license_id: String,
+    pub nats_url: String,
+    pub nats_token: Option<String>,
+    pub heartbeat_seconds: u64,
+    pub log_level: String,
+    pub instances: Vec<InstanceConfig>,
+    pub probe_interval_seconds: u64,
+    pub probe_targets: Vec<ProbeTargetConfig>,
+}
+
+pub fn default_config_path() -> PathBuf {
+    #[cfg(windows)]
+    {
+        PathBuf::from(r"C:\ProgramData\Corrosion\agent.toml")
+    }
+    #[cfg(not(windows))]
+    {
+        PathBuf::from("/etc/corrosion/agent.toml")
+    }
+}
+
+pub fn load(path: &Path) -> Result<Settings> {
+    let raw = std::fs::read_to_string(path)
+        .with_context(|| format!("reading config file {}", path.display()))?;
+    let file: ConfigFile = toml::from_str(&raw)
+        .with_context(|| format!("parsing config file {}", path.display()))?;
+    resolve(file)
+}
+
+/// Merge env overrides (env wins) and validate.
+fn resolve(file: ConfigFile) -> Result<Settings> {
+    let license_id = std::env::var("CORROSION_LICENSE_ID")
+        .ok()
+        .filter(|v| !v.is_empty())
+        .or(file.agent.license_id)
+        .context("license_id missing: set [agent].license_id or CORROSION_LICENSE_ID")?;
+
+    let nats_url = std::env::var("CORROSION_NATS_URL")
+        .ok()
+        .filter(|v| !v.is_empty())
+        .or(file.agent.nats_url)
+        .context("nats_url missing: set [agent].nats_url or CORROSION_NATS_URL")?;
+
+    let nats_token = std::env::var("CORROSION_NATS_TOKEN")
+        .ok()
+        .filter(|v| !v.is_empty())
+        .or(file.agent.nats_token);
+
+    validate_subject_segment("license_id", &license_id)?;
+
+    let mut seen: HashSet<&str> = HashSet::new();
+    for inst in &file.instances {
+        validate_subject_segment("instance id", &inst.id)?;
+        if RESERVED_INSTANCE_IDS.contains(&inst.id.as_str()) {
+            bail!("instance id '{}' is reserved", inst.id);
+        }
+        if !seen.insert(inst.id.as_str()) {
+            bail!("duplicate instance id '{}'", inst.id);
+        }
+        if !SUPPORTED_GAMES.contains(&inst.game.as_str()) {
+            bail!(
+                "instance '{}': unsupported game '{}' (supported: {})",
+                inst.id,
+                inst.game,
+                SUPPORTED_GAMES.join(", ")
+            );
+        }
+    }
+
+    if file.agent.heartbeat_seconds < 10 {
+        bail!("[agent].heartbeat_seconds must be >= 10");
+    }
+
+    Ok(Settings {
+        license_id,
+        nats_url,
+        nats_token,
+        heartbeat_seconds: file.agent.heartbeat_seconds,
+        log_level: file.agent.log_level,
+        instances: file.instances,
+        probe_interval_seconds: file.prober.interval_seconds.max(30),
+        probe_targets: file.prober.targets,
+    })
+}
+
+/// NATS subject segments must not contain '.', '*', '>', whitespace, etc.
+/// Keep it strict: lowercase alphanumerics plus '-' and '_', max 64 chars.
+fn validate_subject_segment(what: &str, value: &str) -> Result<()> {
+    if value.is_empty() || value.len() > 64 {
+        bail!("{what} '{value}' must be 1-64 characters");
+    }
+    if !value
+        .chars()
+        .all(|c| c.is_ascii_lowercase() || c.is_ascii_digit() || c == '-' || c == '_')
+    {
+        bail!("{what} '{value}' may only contain lowercase letters, digits, '-' and '_'");
+    }
+    Ok(())
+}