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/.cargo/config.toml

@@ -0,0 +1,22 @@
+# Corrosion Host Agent — cross-compilation configuration
+#
+# Deploy targets:
+#   Linux:   x86_64-unknown-linux-musl  (fully static — runs on any distro)
+#   Windows: x86_64-pc-windows-msvc     (build via `cargo xwin build` on non-Windows)
+#
+# Prerequisites on macOS:
+#   brew install filosottile/musl-cross/musl-cross   (x86_64-linux-musl-gcc)
+#   cargo install cargo-xwin                          (bundles MSVC CRT + lld-link)
+
+[target.x86_64-unknown-linux-musl]
+linker = "x86_64-linux-musl-gcc"
+
+[env]
+CC_x86_64_unknown_linux_musl = "x86_64-linux-musl-gcc"
+
+[target.x86_64-pc-windows-msvc]
+linker = "lld-link"
+# Statically link the MSVC CRT so the agent runs on fresh Windows installs
+# without the Visual C++ Redistributable (otherwise: STATUS_DLL_NOT_FOUND on
+# any machine missing VCRUNTIME140.dll — most fresh OEM images).
+rustflags = ["-C", "target-feature=+crt-static"]