feat: rewrite envd guest agent in Rust (envd-rs)

Complete Rust rewrite of the Go envd guest daemon that runs as PID 1
inside Firecracker microVMs. Feature-complete across all 8 phases:

- Health, metrics, and env var endpoints
- Crypto (SHA-256/512, HMAC), auth (secure token, signing), init/snapshot
- Connect RPC via connectrpc + buffa (process + filesystem services)
- File transfer (GET/POST /files) with gzip, multipart, chown, ENOSPC
- Port subsystem (/proc/net/tcp scanner, socat forwarder)
- Cgroup2 manager with noop fallback
- Snapshot/restore lifecycle (conntracker, port subsystem stop/restart)
- SIGTERM graceful shutdown, --cmd initial process spawn
- MMDS metadata polling for Firecracker mode

42 source files, ~4200 LOC, 4.1MB stripped release binary.
Makefile updated: build-envd now targets Rust (musl static),
build-envd-go preserved for Go builds.

envd-rs/build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    connectrpc_build::Config::new()
+        .files(&[
+            "../proto/envd/process.proto",
+            "../proto/envd/filesystem.proto",
+        ])
+        .includes(&["../proto/envd", "/usr/include"])
+        .include_file("_connectrpc.rs")
+        .compile()
+        .unwrap();
+}