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/agent.example.toml

@@ -0,0 +1,39 @@
+# Corrosion Host Agent configuration
+# Default location: /etc/corrosion/agent.toml (Linux)
+#                   C:\ProgramData\Corrosion\agent.toml (Windows)
+# Override with: corrosion-host-agent --config /path/to/agent.toml
+#
+# Secrets can come from the environment instead of this file:
+#   CORROSION_LICENSE_ID, CORROSION_NATS_URL, CORROSION_NATS_TOKEN
+
+[agent]
+license_id = "your-license-uuid"
+nats_url = "nats://nats.corrosionmgmt.com:4222"
+# nats_token = "set-me-or-use-CORROSION_NATS_TOKEN"
+heartbeat_seconds = 60
+log_level = "info"
+
+# One agent supervises every game instance on this host.
+# Each instance gets a stable id (lowercase letters, digits, '-', '_') that
+# the panel uses to address it. Changing an id orphans its panel history.
+
+[[instance]]
+id = "rust-main"
+game = "rust"          # rust | conan | soulmask | dune
+root = "/opt/rustserver"
+label = "Main 2x Vanilla"
+
+# [[instance]]
+# id = "soulmask-main"
+# game = "soulmask"
+# root = "/opt/soulmask/main"
+# label = "Cloud Mist Forest (cluster main)"
+
+[prober]
+interval_seconds = 300
+
+# Extra outbound TCP checks beyond the built-in defaults:
+# [[prober.target]]
+# name = "steam-cdn"
+# host = "steamcdn-a.akamaihd.net"
+# port = 443