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

internal/vm/config.go

@@ -70,7 +70,7 @@ func (c *VMConfig) applyDefaults() {
 		c.SocketPath = fmt.Sprintf("/tmp/fc-%s.sock", c.SandboxID)
 	}
 	if c.SandboxDir == "" {
-		c.SandboxDir = fmt.Sprintf("/tmp/fc-sandbox-%s", c.SandboxID)
+		c.SandboxDir = "/tmp/fc-vm"
 	}
 	if c.TapDevice == "" {
 		c.TapDevice = "tap0"