Upgrade to LTX-2.3 with audio generation

- Switch from mlx_video.generate_av to mlx_video.models.ltx_2.generate
- Use prince-canuma/LTX-2.3-distilled model with google/gemma-3-12b-it text encoder
- Add --audio flag for joint audio-video generation
- Add auto-background execution with nohup logging
- Add CLAUDE.md and test stories

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+mlx-video-maker is a toolkit for generating multi-scene AI videos with seamless transitions on Apple Silicon Macs. It uses LTX-2 (2B parameter DiT model) via the MLX framework for native inference on M1/M2/M3/M4 chips.
+
+## How It Works
+
+The core technique is **I2V chaining**: Scene 1 is generated as text-to-video, then the last frame is extracted and used as the input image for scene 2 (image-to-video), and so on. All scenes are concatenated into a final movie with ffmpeg.
+
+Pipeline: T2V (scene 1) → extract last frame → I2V (scene 2+) → repeat → ffmpeg concat
+
+## Running
+
+```bash
+# Generate a movie from a story file
+./generate_story.sh stories/my_story.txt [output_dir]
+
+# Background generation (long runs)
+nohup ./generate_story.sh stories/my_story.txt output/ > output/nohup.out 2>&1 &
+```
+
+### Key parameters (all optional)
+- `--width 1920` (must be divisible by 64)
+- `--height 1088` (must be divisible by 64)
+- `--frames 121` (must satisfy 1 + 8*k)
+- `--strength 0.7` (I2V conditioning, 0.0-1.0; 0.7 is sweet spot)
+- `--fps 24`
+- `--python ./venv/bin/python`
+
+## Architecture
+
+- **`generate_story.sh`** — Single bash script that orchestrates everything. Handles argument parsing, scene generation (calling `mlx_video.generate_av`), frame extraction via ffprobe/ffmpeg, and final concatenation.
+- **`stories/`** — Story files: plain text, one prompt per line, `#` for comments, empty lines ignored.
+- **`promptguide.md`** — Comprehensive LTX-2 prompt engineering guide covering shot establishment, camera movement, audio description, and multi-shot continuity techniques.
+- **`output/`** — Generated artifacts: `scene{N}.mp4`, `scene{N}_lastframe.jpg`, `concat_list.txt`, final movie (gitignored).
+
+## Key Design Decisions
+
+- **Resumable**: Script skips scenes that already exist in the output directory.
+- **No Python package structure**: Direct script execution, single pip dependency (`mlx-video` from GitHub).
+- **No build/test/lint**: Manual testing by reviewing generated video output.
+
+## Requirements
+
+- Apple Silicon Mac with 64GB+ RAM (32GB minimum at lower resolution)
+- Python 3.11+, ffmpeg, ffprobe
+- `pip install git+https://github.com/Blaizzy/mlx-video.git`
+
+## Story File Format
+
+```text
+# Comments start with #
+A cinematic wide shot of a mountain landscape at golden hour...
+
+A close-up tracking shot follows a character walking through fog...
+```
+
+Each non-empty, non-comment line is one scene prompt. Prompts should be flowing narrative paragraphs (not keyword lists) following the six elements in promptguide.md: shot establishment, scene setting, action, character definition, camera movement, audio description.