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/src/rpc/mod.rs

@@ -0,0 +1,26 @@
+pub mod pb;
+pub mod entry;
+pub mod process_handler;
+pub mod process_service;
+pub mod filesystem_service;
+
+use std::sync::Arc;
+
+use crate::rpc::process_service::ProcessServiceImpl;
+use crate::rpc::filesystem_service::FilesystemServiceImpl;
+use crate::state::AppState;
+
+use pb::process::ProcessExt;
+use pb::filesystem::FilesystemExt;
+
+/// Build the connect-rust Router with both RPC services registered.
+pub fn rpc_router(state: Arc<AppState>) -> connectrpc::Router {
+    let process_svc = Arc::new(ProcessServiceImpl::new(Arc::clone(&state)));
+    let filesystem_svc = Arc::new(FilesystemServiceImpl::new(Arc::clone(&state)));
+
+    let router = connectrpc::Router::new();
+    let router = process_svc.register(router);
+    let router = filesystem_svc.register(router);
+
+    router
+}