Improve song metadata handling in `input.harbor` #11907

Job	Run time
build_details	3s
build_opam (4.14.x)	9m 11s
build_js	2m 29s
build_opam (5.2.x)	5m 9s
tree_sitter_parse	47s
build_win32 (x64)	4m 47s
build_no_depopts	5m 34s
lezer_parse	47s
build_posix (debian_trixie, amd64)	15m 17s
build_posix (debian_trixie, arm64)	13m 24s
run_tests (@citest)	19m 28s
build_posix (debian_bookworm, amd64)	14m 45s
update_doc	0s
run_tests (@doctest)	7m 46s
build_posix (debian_bookworm, arm64)	12m 30s
run_tests (@mediatest)	12m 1s
build_posix (ubuntu_oracular, amd64)	14m 37s
build_posix (ubuntu_oracular, arm64)	12m 39s
build_posix (ubuntu_noble, amd64)	14m 39s
build_posix (ubuntu_noble, arm64)	12m 19s
build_posix (alpine, amd64)	10m 22s
build_posix (alpine, arm64)	8m 9s
fetch_s3_artifacts	21s
build_docker (amd64)	2m 26s
build_docker (arm64)	2m 0s
build_docker_minimal	0s
build_docker_alpine_minimal	0s
update_release	0s
build_docker_alpine	0s
build_docker_release	0s
build_docker_release-minimal	0s
	3h 21m 30s

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