Cleanup: crossfade, muxer rewrite, fix add #9615

Job	Run time
build_details	2s
pre-commit	3m 5s
build_js	1m 25s
lezer_parse	49s
build_no_depopts	6m 44s
build_opam (4.14.x)	4m 58s
build_opam (5.0.x)	4m 59s
build_posix (debian_trixie, amd64)	15m 35s
build_win32 (x64)	11m 11s
run_tests (@citest)	19m 14s
build_posix (debian_bookworm, amd64)	14m 34s
run_tests (@doctest)	8m 33s
build_posix (ubuntu_jammy, amd64)	14m 9s
run_tests (@mediatest)	12m 25s
tree_sitter_parse	58s
build_posix (ubuntu_lunar, amd64)	14m 9s
update_doc	0s
build_posix (alpine, amd64)	10m 16s
fetch_s3_artifacts	23s
build_docker_minimal	0s
update_release	0s
build_docker	0s
build_docker_alpine	0s
build_docker_alpine_minimal	0s
build_docker_release	0s
build_docker_release-minimal	0s
	2h 23m 29s

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