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>

tests/test_build_pool_manifest.py

@@ -54,3 +54,17 @@ def test_generate_media_bakes_every_member_pair(monkeypatch, tmp_path):
            for h in b.POOLS[H] for l in b.POOLS[L]}
     assert fwd <= outs and rev <= outs
     assert len(fwd) + len(rev) == 154
+
+
+def test_transition_cmd_is_all_intra():
+    cmd = b.transition_cmd("ffmpeg", "/a/base.mp4", "/b/base.mp4", "/out/x__y.mp4")
+    assert "-g" in cmd and cmd[cmd.index("-g") + 1] == "1"
+    assert "-keyint_min" in cmd and cmd[cmd.index("-keyint_min") + 1] == "1"
+    assert "-sc_threshold" in cmd and cmd[cmd.index("-sc_threshold") + 1] == "0"
+    assert "libx264" in cmd
+
+
+def test_reverse_cmd_is_all_intra():
+    cmd = b.reverse_cmd("ffmpeg", "/out/x__y.mp4", "/out/x__y.rev.mp4")
+    assert "-g" in cmd and cmd[cmd.index("-g") + 1] == "1"
+    assert "reverse" in " ".join(cmd)