Cleanup: crossfade, muxer rewrite, fix add #9616

Job	Run time
build_js	1m 24s
pre-commit	3m 2s
build_details	4s
lezer_parse	48s
tree_sitter_parse	51s
build_no_depopts	5m 50s
run_tests (@citest)	13m 1s
build_posix (debian_trixie, amd64)	15m 8s
run_tests (@doctest)	8m 28s
build_opam (4.14.x)	4m 43s
build_posix (debian_bookworm, amd64)	15m 40s
build_win32 (x64)	10m 53s
run_tests (@mediatest)	11m 57s
build_opam (5.0.x)	4m 56s
build_posix (ubuntu_jammy, amd64)	14m 29s
build_posix (ubuntu_lunar, amd64)	14m 57s
build_posix (alpine, amd64)	11m 14s
update_doc	0s
fetch_s3_artifacts	23s
build_docker_alpine (amd64)	2m 22s
build_docker_minimal (amd64)	2m 15s
update_release	0s
build_docker (amd64)	2m 27s
build_docker_alpine_minimal (amd64)	44s
build_docker_release-minimal	0s
build_docker_release	0s
	2h 25m 36s

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