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/subjects.rs

@@ -0,0 +1,30 @@
+//! Corrosion wire protocol v2 subject scheme (see PROTOCOL.md).
+//!
+//! Host-level subjects live under `corrosion.{license}.host.*`; per-instance
+//! subjects under `corrosion.{license}.{instance_id}.*`. Instance ids are
+//! validated at config load so they can never collide with the reserved
+//! `host` segment or contain subject metacharacters.
+
+pub fn host_heartbeat(license: &str) -> String {
+    format!("corrosion.{license}.host.heartbeat")
+}
+
+pub fn host_cmd(license: &str) -> String {
+    format!("corrosion.{license}.host.cmd")
+}
+
+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)]
+pub fn instance_cmd(license: &str, instance: &str) -> String {
+    format!("corrosion.{license}.{instance}.cmd")
+}
+
+/// Phase 1: per-instance state-change events.
+#[allow(dead_code)]
+pub fn instance_status(license: &str, instance: &str) -> String {
+    format!("corrosion.{license}.{instance}.status")
+}