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/state.rs

@@ -0,0 +1,42 @@
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+use crate::auth::token::SecureToken;
+use crate::conntracker::ConnTracker;
+use crate::execcontext::Defaults;
+use crate::port::subsystem::PortSubsystem;
+use crate::util::AtomicMax;
+
+pub struct AppState {
+    pub defaults: Defaults,
+    pub version: String,
+    pub commit: String,
+    pub is_fc: bool,
+    pub needs_restore: AtomicBool,
+    pub last_set_time: AtomicMax,
+    pub access_token: SecureToken,
+    pub conn_tracker: ConnTracker,
+    pub port_subsystem: Option<Arc<PortSubsystem>>,
+}
+
+impl AppState {
+    pub fn new(
+        defaults: Defaults,
+        version: String,
+        commit: String,
+        is_fc: bool,
+        port_subsystem: Option<Arc<PortSubsystem>>,
+    ) -> Arc<Self> {
+        Arc::new(Self {
+            defaults,
+            version,
+            commit,
+            is_fc,
+            needs_restore: AtomicBool::new(false),
+            last_set_time: AtomicMax::new(),
+            access_token: SecureToken::new(),
+            conn_tracker: ConnTracker::new(),
+            port_subsystem,
+        })
+    }
+}