no behaviour change: a pure move of items and tests into new modules, plus
visibility edits (pub(crate)) and path updates in sibling modules that named
crate-root items. same routes, same OpenAPI, 561 tests before and after.
why: #5470, new features stop landing in one 6.4k-line file by default.
files outside swarm-controller/src: docs/agent-lifecycle/agent-roster.md,
swarm-ui JobsPage.tsx and AgentsPage.tsx, nix/host-modules/swarm-controller.nix
and swarm-grafana.nix hold comments that named main.rs for items that moved.
