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/permissions/mod.rs

@@ -0,0 +1,2 @@
+pub mod user;
+pub mod path;

envd-rs/src/permissions/path.rs

@@ -0,0 +1,72 @@
+use std::fs;
+use std::os::unix::fs::chown;
+use std::path::{Path, PathBuf};
+
+use nix::unistd::{Gid, Uid};
+
+fn expand_tilde(path: &str, home_dir: &str) -> Result<String, String> {
+    if path.is_empty() || !path.starts_with('~') {
+        return Ok(path.to_string());
+    }
+    if path.len() > 1 && path.as_bytes()[1] != b'/' && path.as_bytes()[1] != b'\\' {
+        return Err("cannot expand user-specific home dir".into());
+    }
+    Ok(format!("{}{}", home_dir, &path[1..]))
+}
+
+pub fn expand_and_resolve(
+    path: &str,
+    home_dir: &str,
+    default_path: Option<&str>,
+) -> Result<String, String> {
+    let path = if path.is_empty() {
+        default_path.unwrap_or("").to_string()
+    } else {
+        path.to_string()
+    };
+
+    let path = expand_tilde(&path, home_dir)?;
+
+    if Path::new(&path).is_absolute() {
+        return Ok(path);
+    }
+
+    let joined = PathBuf::from(home_dir).join(&path);
+    joined
+        .canonicalize()
+        .or_else(|_| Ok(joined))
+        .map(|p| p.to_string_lossy().to_string())
+}
+
+pub fn ensure_dirs(path: &str, uid: Uid, gid: Gid) -> Result<(), String> {
+    let path = Path::new(path);
+    let mut current = PathBuf::new();
+
+    for component in path.components() {
+        current.push(component);
+        let current_str = current.to_string_lossy();
+
+        if current_str == "/" {
+            continue;
+        }
+
+        match fs::metadata(&current) {
+            Ok(meta) => {
+                if !meta.is_dir() {
+                    return Err(format!("path is a file: {current_str}"));
+                }
+            }
+            Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+                fs::create_dir(&current)
+                    .map_err(|e| format!("failed to create directory {current_str}: {e}"))?;
+                chown(&current, Some(uid.as_raw()), Some(gid.as_raw()))
+                    .map_err(|e| format!("failed to chown directory {current_str}: {e}"))?;
+            }
+            Err(e) => {
+                return Err(format!("failed to stat directory {current_str}: {e}"));
+            }
+        }
+    }
+
+    Ok(())
+}

envd-rs/src/permissions/user.rs

@@ -0,0 +1,32 @@
+use nix::unistd::{Gid, Group, Uid, User};
+
+pub fn lookup_user(username: &str) -> Result<User, String> {
+    User::from_name(username)
+        .map_err(|e| format!("error looking up user '{username}': {e}"))?
+        .ok_or_else(|| format!("user '{username}' not found"))
+}
+
+pub fn get_uid_gid(user: &User) -> (Uid, Gid) {
+    (user.uid, user.gid)
+}
+
+pub fn get_user_groups(user: &User) -> Vec<Gid> {
+    let c_name = std::ffi::CString::new(user.name.as_str()).unwrap();
+    nix::unistd::getgrouplist(&c_name, user.gid).unwrap_or_default()
+}
+
+pub fn lookup_username_by_uid(uid: Uid) -> String {
+    User::from_uid(uid)
+        .ok()
+        .flatten()
+        .map(|u| u.name)
+        .unwrap_or_else(|| uid.to_string())
+}
+
+pub fn lookup_groupname_by_gid(gid: Gid) -> String {
+    Group::from_gid(gid)
+        .ok()
+        .flatten()
+        .map(|g| g.name)
+        .unwrap_or_else(|| gid.to_string())
+}