add high quality video export and wrapper script

- replace diffusers export_to_video with direct imageio writer
- add --crf option for quality control (default 18, visually lossless)
- use libx264 with slow preset for better compression
- add ltx2 wrapper script for easier cli usage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ltx2

@@ -0,0 +1,6 @@
+#!/bin/bash
+# quick wrapper for ltx-2 generation
+
+source ~/ltx-venv/bin/activate
+cd "$(dirname "$0")"
+python generate.py "$@"