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/cgroups/mod.rs

@@ -0,0 +1,66 @@
+use std::collections::HashMap;
+use std::fs;
+use std::os::unix::io::{OwnedFd, RawFd};
+use std::path::PathBuf;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum ProcessType {
+    Pty,
+    User,
+    Socat,
+}
+
+pub trait CgroupManager: Send + Sync {
+    fn get_fd(&self, proc_type: ProcessType) -> Option<RawFd>;
+}
+
+pub struct Cgroup2Manager {
+    fds: HashMap<ProcessType, OwnedFd>,
+}
+
+impl Cgroup2Manager {
+    pub fn new(root: &str, configs: &[(ProcessType, &str, &[(&str, &str)])]) -> Result<Self, String> {
+        let mut fds = HashMap::new();
+
+        for (proc_type, sub_path, properties) in configs {
+            let full_path = PathBuf::from(root).join(sub_path);
+
+            fs::create_dir_all(&full_path).map_err(|e| {
+                format!("failed to create cgroup {}: {e}", full_path.display())
+            })?;
+
+            for (name, value) in *properties {
+                let prop_path = full_path.join(name);
+                fs::write(&prop_path, value).map_err(|e| {
+                    format!("failed to write cgroup property {}: {e}", prop_path.display())
+                })?;
+            }
+
+            let fd = nix::fcntl::open(
+                &full_path,
+                nix::fcntl::OFlag::O_RDONLY,
+                nix::sys::stat::Mode::empty(),
+            )
+            .map_err(|e| format!("failed to open cgroup {}: {e}", full_path.display()))?;
+
+            fds.insert(*proc_type, fd);
+        }
+
+        Ok(Self { fds })
+    }
+}
+
+impl CgroupManager for Cgroup2Manager {
+    fn get_fd(&self, proc_type: ProcessType) -> Option<RawFd> {
+        use std::os::unix::io::AsRawFd;
+        self.fds.get(&proc_type).map(|fd| fd.as_raw_fd())
+    }
+}
+
+pub struct NoopCgroupManager;
+
+impl CgroupManager for NoopCgroupManager {
+    fn get_fd(&self, _proc_type: ProcessType) -> Option<RawFd> {
+        None
+    }
+}