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/http/snapshot.rs

@@ -0,0 +1,32 @@
+use std::sync::Arc;
+use std::sync::atomic::Ordering;
+
+use axum::extract::State;
+use axum::http::{StatusCode, header};
+use axum::response::IntoResponse;
+
+use crate::state::AppState;
+
+/// POST /snapshot/prepare — quiesce subsystems before Firecracker snapshot.
+///
+/// In Rust there is no GC dance. We just:
+/// 1. Stop port subsystem
+/// 2. Close idle connections via conntracker
+/// 3. Set needs_restore flag
+pub async fn post_snapshot_prepare(State(state): State<Arc<AppState>>) -> impl IntoResponse {
+    if let Some(ref ps) = state.port_subsystem {
+        ps.stop();
+        tracing::info!("snapshot/prepare: port subsystem stopped");
+    }
+
+    state.conn_tracker.prepare_for_snapshot();
+    tracing::info!("snapshot/prepare: connections prepared");
+
+    state.needs_restore.store(true, Ordering::Release);
+    tracing::info!("snapshot/prepare: ready for freeze");
+
+    (
+        StatusCode::NO_CONTENT,
+        [(header::CACHE_CONTROL, "no-store")],
+    )
+}