Add sandbox snapshot and restore with UFFD lazy memory loading

Implement full snapshot lifecycle: pause (snapshot + free resources),
resume (UFFD-based lazy restore), and named snapshot templates that
can spawn new sandboxes from frozen VM state.

Key changes:
- Snapshot header system with generational diff mapping (inspired by e2b)
- UFFD server for lazy page fault handling during snapshot restore
- Stable rootfs symlink path (/tmp/fc-vm/) for snapshot compatibility
- Templates DB table and CRUD API endpoints (POST/GET/DELETE /v1/snapshots)
- CreateSnapshot/DeleteSnapshot RPCs in hostagent proto
- Reconciler excludes paused sandboxes (expected absent from host agent)
- Snapshot templates lock vcpus/memory to baked-in values
- Proper cleanup of uffd sockets and pause snapshot files on destroy

.env.example

@@ -10,6 +10,7 @@ AGENT_LISTEN_ADDR=:50051
 AGENT_KERNEL_PATH=/var/lib/wrenn/kernels/vmlinux
 AGENT_IMAGES_PATH=/var/lib/wrenn/images
 AGENT_SANDBOXES_PATH=/var/lib/wrenn/sandboxes
+AGENT_SNAPSHOTS_PATH=/var/lib/wrenn/snapshots
 AGENT_HOST_INTERFACE=eth0
 
 # Lago (billing — external service)