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properly formatted parameter docs, and an example showing how to
implement modular services for a new containing system.
Move the portable modular service base from
nixos/modules/system/service/portable/ to lib/services/, making it
importable by any module system (home-manager, nix-darwin) without
reaching into the nixos/ tree.
Moved files: service.nix, lib.nix, config-data.nix, config-data-item.nix,
test.nix. All external references updated (systemd/system.nix,
doc/manual/default.nix, assertions.nix, README.md).
No functional changes - only import paths differ.
