feat: ffmpeg frame extraction + optional dominant_color

Per sub-project-2 plan Task 5 / spec §5.2. ffmpeg-only dominant color (no image
library), opt-in by design; extract_frame for review previews. Runners injectable
so the unit suite never shells out.

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

tests/test_mediatools.py

@@ -0,0 +1,38 @@
+from tools.mediatools import (
+    compute_dominant_color,
+    dominant_color_from_rgb,
+    extract_frame,
+)
+
+
+def test_dominant_color_from_rgb_pure_red():
+    assert dominant_color_from_rgb(b"\xff\x00\x00") == "#ff0000"
+
+
+def test_dominant_color_from_rgb_arbitrary():
+    assert dominant_color_from_rgb(b"\x12\xab\x0f") == "#12ab0f"
+
+
+def test_compute_dominant_color_uses_runner():
+    captured = {}
+
+    def fake_runner(args):
+        captured["args"] = args
+        return b"\x00\x80\xff"
+
+    color = compute_dominant_color("clip.mp4", midpoint_s=5.0, runner=fake_runner)
+    assert color == "#0080ff"
+    assert "ffmpeg" in captured["args"]
+    assert "5.0" in captured["args"]
+
+
+def test_extract_frame_invokes_runner_and_returns_dest(tmp_path):
+    dest = tmp_path / "frame.png"
+    calls = []
+
+    def fake_runner(args):
+        calls.append(args)
+
+    out = extract_frame("clip.mp4", dest, midpoint_s=2.0, runner=fake_runner)
+    assert out == dest
+    assert calls and "ffmpeg" in calls[0] and str(dest) in calls[0]