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/envs.rs

@@ -0,0 +1,25 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use axum::Json;
+use axum::extract::State;
+use axum::http::header;
+use axum::response::IntoResponse;
+
+use crate::state::AppState;
+
+pub async fn get_envs(State(state): State<Arc<AppState>>) -> impl IntoResponse {
+    tracing::debug!("getting env vars");
+
+    let envs: HashMap<String, String> = state
+        .defaults
+        .env_vars
+        .iter()
+        .map(|entry| (entry.key().clone(), entry.value().clone()))
+        .collect();
+
+    (
+        [(header::CACHE_CONTROL, "no-store")],
+        Json(envs),
+    )
+}