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

CLAUDE.md

@@ -209,7 +209,7 @@ The main module (`go.mod`) and envd (`envd/go.mod`) are fully independent. `make
 - Kernel: `/var/lib/wrenn/kernels/vmlinux`
 - Base rootfs images: `/var/lib/wrenn/images/{template}.ext4`
 - Sandbox clones: `/var/lib/wrenn/sandboxes/`
-- Firecracker: `/usr/local/bin/firecracker`
+- Firecracker: `/usr/local/bin/firecracker` (e2b's fork of firecracker)
 
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