Add SEQUENCE node handling for concatenating clips

Uses FFmpeg concat demuxer. Falls back to re-encoding if
stream copy fails (different codecs/formats).

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

legacy_tasks.py

@@ -843,6 +843,75 @@ def execute_recipe(self, recipe_sexp: str, input_hashes: Dict[str, str], run_id:
                     logger.info(f"COMPOUND step {step.step_id}: {len(filter_chain)} effects -> {content_cid[:16]}...")
                     continue
 
+                # Handle SEQUENCE nodes (concatenate clips)
+                if step.node_type.upper() == "SEQUENCE":
+                    import subprocess
+                    import tempfile
+
+                    if len(input_paths) < 2:
+                        raise ValueError(f"SEQUENCE requires at least 2 inputs, got {len(input_paths)}")
+
+                    # Create concat list file for FFmpeg
+                    temp_dir = Path(tempfile.mkdtemp())
+                    concat_list = temp_dir / "concat.txt"
+                    with open(concat_list, "w") as f:
+                        for inp in input_paths:
+                            f.write(f"file '{inp}'\n")
+
+                    output_dir = CACHE_DIR / "nodes" / step.cache_id
+                    output_dir.mkdir(parents=True, exist_ok=True)
+                    final_output = output_dir / "output.mkv"
+
+                    # FFmpeg concat demuxer
+                    cmd = [
+                        "ffmpeg", "-y",
+                        "-f", "concat",
+                        "-safe", "0",
+                        "-i", str(concat_list),
+                        "-c", "copy",
+                        str(final_output)
+                    ]
+
+                    logger.info(f"SEQUENCE: Concatenating {len(input_paths)} clips")
+                    result = subprocess.run(cmd, capture_output=True, text=True)
+
+                    if result.returncode != 0:
+                        # Try with re-encoding if copy fails
+                        cmd = [
+                            "ffmpeg", "-y",
+                            "-f", "concat",
+                            "-safe", "0",
+                            "-i", str(concat_list),
+                            "-c:v", "libx264", "-c:a", "aac",
+                            str(final_output)
+                        ]
+                        result = subprocess.run(cmd, capture_output=True, text=True)
+                        if result.returncode != 0:
+                            raise RuntimeError(f"FFmpeg concat failed: {result.stderr}")
+
+                    # Upload to IPFS
+                    cached, content_cid = cache_manager.put(
+                        final_output,
+                        node_type="SEQUENCE",
+                        node_id=step.cache_id,
+                    )
+
+                    # Cleanup
+                    import shutil
+                    shutil.rmtree(temp_dir, ignore_errors=True)
+
+                    step_results[step.step_id] = {
+                        "status": "executed",
+                        "path": str(final_output),
+                        "cache_id": step.cache_id,
+                        "cid": content_cid,
+                        "input_count": len(input_paths),
+                    }
+                    cache_id_to_path[step.cache_id] = final_output
+                    total_executed += 1
+                    logger.info(f"SEQUENCE step {step.step_id}: {len(input_paths)} clips -> {content_cid[:16]}...")
+                    continue
+
                 # Get executor for this step type
                 executor = get_executor(step.node_type)
                 if not executor: