feat(pipeline): all-intra ffmpeg builders for seekable morph re-bake

Per plan Task 7. Extracts transition_cmd/reverse_cmd argv builders and adds the
ALL_INTRA flag set (-g 1 -keyint_min 1 -sc_threshold 0) so every baked frame is a
keyframe — smooth arbitrary-frame seeking for the scrub interaction. Unit-tested.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

simulator/build_pool_manifest.py

@@ -399,6 +399,28 @@ def _adjacent_edges() -> list[tuple[str, str]]:
     return [(RING_ORDER[i], RING_ORDER[(i + 1) % n]) for i in range(n)]
 
 
+# All-intra H.264: every frame a keyframe, so the scrub interaction can seek to an
+# arbitrary frame smoothly (a sparse GOP scrubs "steppy"). Files grow, but each clip
+# stays well under the LFS/proxy ceiling. (Scrub-driven-transitions design §4.)
+ALL_INTRA = ["-c:v", "libx264", "-g", "1", "-keyint_min", "1", "-sc_threshold", "0", "-pix_fmt", "yuv420p"]
+
+
+def transition_cmd(ff: str, a: str, b: str, out: str) -> list[str]:
+    """The ffmpeg argv for a forward (zoom-in) member-pair morph, baked all-intra."""
+    norm = "trim=0:3,setpts=PTS-STARTPTS,scale=1280:720,fps=25,setsar=1,format=yuv420p"
+    return [
+        ff, "-y", "-i", str(a), "-i", str(b), "-filter_complex",
+        f"[0:v]{norm}[a];[1:v]{norm}[b];"
+        "[a][b]xfade=transition=zoomin:duration=1.5:offset=0.75,format=yuv420p[v]",
+        "-map", "[v]", "-an", *ALL_INTRA, str(out),
+    ]
+
+
+def reverse_cmd(ff: str, forward: str, out: str) -> list[str]:
+    """The ffmpeg argv for the zoom-OUT companion (forward played backward), all-intra."""
+    return [ff, "-y", "-i", str(forward), "-vf", "reverse", "-an", *ALL_INTRA, str(out)]
+
+
 def _make_transition(ff: str, src_clip: str, dst_clip: str) -> Path:
     """A zoom/warp morph between two CLIP MEMBERS' base footage (the well-liked
     orbit-coast recipe), keyed by clip id so every adjacent member pair gets its
@@ -407,13 +429,7 @@ def _make_transition(ff: str, src_clip: str, dst_clip: str) -> Path:
     b = MEDIA / dst_clip / "base.mp4"
     out = MEDIA / "transitions" / f"{src_clip}__{dst_clip}.mp4"
     out.parent.mkdir(parents=True, exist_ok=True)
-    norm = "trim=0:3,setpts=PTS-STARTPTS,scale=1280:720,fps=25,setsar=1,format=yuv420p"
-    subprocess.run([
-        ff, "-y", "-i", str(a), "-i", str(b), "-filter_complex",
-        f"[0:v]{norm}[a];[1:v]{norm}[b];"
-        "[a][b]xfade=transition=zoomin:duration=1.5:offset=0.75,format=yuv420p[v]",
-        "-map", "[v]", "-an", str(out),
-    ], check=True, capture_output=True)
+    subprocess.run(transition_cmd(ff, str(a), str(b), str(out)), check=True, capture_output=True)
     return out
 
 
@@ -423,9 +439,7 @@ def _make_reverse(ff: str, forward: Path) -> Path:
     crosses its edge `reversed`; the renderer then plays this file, so zooming out
     recedes from the current scale back to the higher one instead of zooming in."""
     out = forward.with_suffix(".rev.mp4")
-    subprocess.run([
-        ff, "-y", "-i", str(forward), "-vf", "reverse", "-an", str(out),
-    ], check=True, capture_output=True)
+    subprocess.run(reverse_cmd(ff, str(forward), str(out)), check=True, capture_output=True)
     return out