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/PROTOCOL.md

@@ -0,0 +1,143 @@
+# Corrosion Wire Protocol v2
+
+Status: **Phase 0 implemented** (host heartbeat, host commands, going-offline
+beacon). Per-instance command/status subjects are reserved and specified here
+for Phase 1.
+
+## Design
+
+One **host agent** per machine supervises **N game instances**. Subjects are
+scoped license-first, then by addressee:
+
+```
+corrosion.{license_id}.host.*           host-level (the agent itself)
+corrosion.{license_id}.{instance_id}.*  instance-level (one game server)
+```
+
+`instance_id` is a config-defined slug (`[a-z0-9_-]{1,64}`), validated at
+agent start. `host` is a reserved segment and can never be an instance id.
+Payloads are JSON. Every heartbeat carries `"schema": 2` so consumers can
+distinguish v2 from the legacy Go companion protocol (which used
+`corrosion.{license_id}.companion.heartbeat`, no schema field).
+
+## Host-level subjects (Phase 0 — live)
+
+### `corrosion.{license_id}.host.heartbeat` (agent → backend, publish)
+
+Published every `heartbeat_seconds` (default 60, jittered ±20%).
+
+```json
+{
+  "schema": 2,
+  "timestamp": "2026-06-11T18:00:00Z",
+  "agent": {
+    "version": "2.0.0-alpha.1",
+    "commit": "a8722a7",
+    "os": "linux",
+    "arch": "x86_64",
+    "uptime_seconds": 86400
+  },
+  "host": {
+    "hostname": "asgard-01",
+    "cpu_percent": 12.5,
+    "cpu_cores": 80,
+    "mem_total_mb": 262144,
+    "mem_used_mb": 81920,
+    "uptime_seconds": 1209600,
+    "disks": [
+      { "mount": "/", "total_mb": 1907729, "free_mb": 1532211 }
+    ]
+  },
+  "instances": [
+    {
+      "id": "rust-main",
+      "game": "rust",
+      "label": "Main 2x Vanilla",
+      "state": "configured",
+      "root_disk_free_mb": 1532211
+    }
+  ],
+  "probe": {
+    "timestamp": "2026-06-11T17:58:00Z",
+    "results": [
+      { "name": "corrosion-cdn", "host": "cdn.corrosionmgmt.com", "port": 443, "ok": true, "latency_ms": 18 }
+    ]
+  }
+}
+```
+
+All telemetry is measured, never fabricated. Fields the agent cannot measure
+are omitted (`probe` before the first probe completes, `hostname` if
+unavailable).
+
+Phase 0 instance `state` values: `configured` (root path exists),
+`missing_root`. Phase 1 adds live process states: `running`, `stopped`,
+`crashed`, `starting`, `updating`.
+
+### `corrosion.{license_id}.host.cmd` (backend → agent, request-reply)
+
+Request: `{ "func": "<name>" }`. Reply: `{ "status": "success" | "error", ... }`.
+
+| func      | Reply payload                                            |
+| --------- | -------------------------------------------------------- |
+| `ping`    | `version`, `commit`, `uptime_seconds`                     |
+| `probe`   | `report` — fresh ProbeReport (also cached for heartbeat)  |
+| `sysinfo` | `snapshot` — full heartbeat payload, collected on demand  |
+
+Unknown funcs return `status: "error"` with a message listing supported funcs.
+
+### `corrosion.{license_id}.host.going_offline` (agent → backend, publish)
+
+Best-effort beacon (500ms budget) on graceful shutdown so the panel can flip
+the host to offline immediately instead of waiting out heartbeat staleness.
+Payload: `{}`.
+
+## Instance-level subjects (Phase 1 — reserved, not yet implemented)
+
+### `corrosion.{license_id}.{instance_id}.cmd` (backend → agent, request-reply)
+
+Lifecycle and control for one game instance. Planned funcs: `start`, `stop`,
+`restart`, `status`, `rcon` (process-class games), `steam_update`,
+`oxide_install` (rust), plus game-adapter-specific commands (Dune: docker
+lifecycle, RabbitMQ bus commands, Coriolis reset).
+
+### `corrosion.{license_id}.{instance_id}.status` (agent → backend, publish)
+
+State-change events (started/stopped/crashed) so the panel does not wait for
+the next heartbeat.
+
+### `corrosion.{license_id}.{instance_id}.console` (agent → backend, publish)
+
+Live console/log lines for the panel console view.
+
+### `corrosion.{license_id}.{instance_id}.files.cmd` (backend → agent, request-reply)
+
+VueFinder-style file manager ops, jailed to the instance root. Carries over
+the Go agent's jailed filemanager semantics (`fm_list`, `fm_save`, ...); the
+legacy UNJAILED `files.get/put/delete/list` API is retired and will not be
+ported.
+
+## Backend mapping notes (Phase 0)
+
+- The NestJS NATS bridge subscribes `corrosion.*.host.heartbeat` and
+  `corrosion.*.host.going_offline`.
+- Until the license→host→instance schema lands, the backend may map the host
+  heartbeat onto the existing single `server_connections` row per license:
+  `companion_last_seen` ← heartbeat arrival, `connection_status` ←
+  connected/offline, resources ← `host.cpu_percent` / `mem_*` / first disk.
+  Instance-level mapping activates with the fleet schema.
+
+## Probing — scope honesty
+
+The Phase 0 prober measures **outbound** reachability from the host (TCP
+connect + latency). It cannot verify **inbound** port-forwarding (the thing
+players hit). Inbound verification requires a backend-side reverse probe
+service that attempts connections to the customer's public IP/ports on
+request; that is specified as a Phase 1+ feature and will reuse this report
+format with `direction: "inbound"`.
+
+## Versioning
+
+- The agent embeds semver + git hash + build timestamp (`--version`,
+  heartbeat `agent` block).
+- Schema changes bump `schema` and are additive where possible.