feat(pipeline): frame stage ffmpeg arg builder (content pipeline §3.2)

tools/pipeline/__init__.py

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+"""Content pipeline: source -> frame -> loop -> transcode -> manifest.
+
+Pure ffmpeg-argument builders (ffmpeg_ops), a thin runner (run), provenance
+records, and manifest assembly. See docs/superpowers/specs/2026-06-24-content-pipeline-design.md.
+"""

tools/pipeline/ffmpeg_ops.py

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+"""Pure ffmpeg argument builders (no I/O) for the content pipeline.
+
+Each function returns a list[str] ready for ffmpeg, so command construction is
+unit-testable without running ffmpeg. tools/pipeline/run.py executes them. The
+ffmpeg binary name is injected (default "ffmpeg").
+"""
+
+from __future__ import annotations
+
+
+def _fit(width: int, height: int) -> str:
+    """A scale+pad filter chain fitting any input into width×height (letterbox),
+    fixing SAR and pixel format so downstream concat/xfade get identical geometry."""
+    return (
+        f"scale={width}:{height}:force_original_aspect_ratio=decrease,"
+        f"pad={width}:{height}:(ow-iw)/2:(oh-ih)/2,"
+        "setsar=1,format=yuv420p"
+    )
+
+
+def frame_args(src, dst, *, start: float, duration: float,
+               width: int, height: int, ff: str = "ffmpeg") -> list[str]:
+    """Stage 2 — trim [start, start+duration], fit to width×height, strip audio.
+    Produces a high-quality mezzanine for the loop stage."""
+    return [
+        ff, "-y", "-ss", str(start), "-t", str(duration),
+        "-i", str(src), "-vf", _fit(width, height), "-an",
+        "-c:v", "libx264", "-preset", "medium", "-crf", "16",
+        "-pix_fmt", "yuv420p", "-movflags", "+faststart", str(dst),
+    ]