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/README.md

@@ -0,0 +1,36 @@
+# Corrosion Host Agent
+
+Rust rewrite of the Go companion agent (`companion-agent/`, retained as the
+behavior reference until parity). One agent per machine supervises every game
+instance on that host — Rust, Conan Exiles, Soulmask, Dune: Awakening.
+
+- **Wire protocol**: see [PROTOCOL.md](./PROTOCOL.md) (v2, instance-scoped subjects)
+- **Config**: see [agent.example.toml](./agent.example.toml)
+
+## Status — Phase 0
+
+- [x] Multi-instance TOML config + env overrides (`CORROSION_LICENSE_ID`, `CORROSION_NATS_URL`, `CORROSION_NATS_TOKEN`)
+- [x] NATS connection (infinite reconnect, capped backoff, 30s ping, offline send-buffering, `tls://` support)
+- [x] Host heartbeat with real telemetry (sysinfo: CPU, memory, disks) — no fabricated values
+- [x] Connectivity prober (outbound TCP, periodic + on-demand)
+- [x] Host command channel (`ping`, `probe`, `sysinfo`)
+- [x] Graceful shutdown (cancellation token, going-offline beacon, NATS flush)
+- [ ] Phase 1: process-class game adapter (spawn/RCON/SteamCMD/files) — Rust, Conan, Soulmask
+- [ ] Phase 2: Dune Docker adapter (compose lifecycle, RabbitMQ bus, Postgres admin)
+- [ ] Phase 3: signed self-update (enforced ed25519 — release gate), service install, supervisor split
+
+## Build
+
+```bash
+cargo build --release                                    # native
+cargo build --release --target x86_64-unknown-linux-gnu  # linux deploy target
+cargo build --release --target x86_64-pc-windows-msvc    # windows (cargo-xwin on non-Windows)
+```
+
+## Run
+
+```bash
+corrosion-host-agent --config ./agent.toml         # foreground
+corrosion-host-agent --config ./agent.toml check   # validate config only
+corrosion-host-agent version                       # semver + git hash + build ts
+```