test: add integration tests with real media for music video pipeline

Build real PNG, WAV, and MP4 fixtures (no AI models) and exercise the
full assembler and Creative Director pipeline end-to-end.  Fix MoviePy v2
crossfade API (vfx.CrossFadeIn) and font resolution (DejaVu-Sans).

14 new integration tests — 638 total, all passing.

https://claude.ai/code/session_01KJm6jQkNi3aA3yoQJn636c

pyproject.toml

@@ -91,7 +91,7 @@ include = [
 
 [tool.pytest.ini_options]
 testpaths = ["tests"]
-pythonpath = ["src"]
+pythonpath = ["src", "tests"]
 asyncio_mode = "auto"
 asyncio_default_fixture_loop_scope = "function"
 addopts = "-v --tb=short"