fix: accurate sandbox metrics and memory management

Three issues fixed:

1. Memory metrics read host-side VmRSS of the Firecracker process,
   which includes guest page cache and never decreases. Replaced
   readMemRSS(fcPID) with readEnvdMemUsed(client) that queries
   envd's /metrics endpoint for guest-side total - MemAvailable.
   This matches neofetch and reflects actual process memory.

2. Added Firecracker balloon device (deflate_on_oom, 5s stats) and
   envd-side periodic page cache reclaimer (drop_caches when >80%
   used). Reclaimer is gated by snapshot_in_progress flag with
   sync() before freeze to prevent memory corruption during pause.

3. Sampling interval 500ms → 1s, ring buffer capacities adjusted
   to maintain same time windows. Reduces per-host HTTP load from
   240 calls/sec to 120 calls/sec at 120 capsules.

Also: maxDiffGenerations 8 → 1 (merge every re-pause since UFFD
lazy-loads anyway), envd mem_used formula uses total - available.

envd-rs/src/http/init.rs

@@ -147,6 +147,9 @@ async fn trigger_restore_and_respond(state: &AppState) -> axum::response::Respon
 
 fn post_restore_recovery(state: &AppState) {
     tracing::info!("restore: post-restore recovery (no GC needed in Rust)");
+
+    state.snapshot_in_progress.store(false, std::sync::atomic::Ordering::Release);
+
     state.conn_tracker.restore_after_snapshot();
 
     if let Some(ref ps) = state.port_subsystem {