[clhodapp]

Builds on nix-caisson/caisson-core#54, merged, which records defaultEcosystemSrcNames on the lib manifest. The locks pin caisson-core main at its merge commit, 68fe584.

Every configuration records ecosystems: an account for each part of the stack its evaluation used, keyed as the parts are resolved. This is item 4 of what topside needs from the record, and design section 5 lists it.

{
  nixpkgs-lib = { from = "defaultEcosystemSrc"; source = "nixpkgs"; };
  nixos = { from = "ecosystemSrc"; source = "nixpkgs"; };
}

• from names the declaration that supplied the source: ecosystemSrc for the configuration's selection, defaultEcosystemSrc for the tree's, and sources for the source pinned under exactly that name.
• source is the name of that source among the pinned sources, where sources.<name>.pin records its revision. It is null where a selection builds the value in another way.
• lib.caisson.integrations.ecosystemAccountOf computes an account from the arguments resolveEcosystemSrc takes. mkModuleConfiguration records nixpkgs-lib for every evaluation, and each integration's evaluate returns the ecosystems it resolves, with the same arguments it resolves them with. Every integration that resolves a source records it: nixos and nixos-minimal, home-manager, darwin, system-manager, colmena and caisson-colmena, disko and caisson-disko, hjem, nixvim, nvf, nixidy, treefmt, flake-parts, kubenix, topology and terranix.
• A package config records nixpkgs-lib and nixpkgs. Its nixpkgs entry also lists libraries, the top-level names its sets take from the package set's source.

A fix in flake-parts. Since #128, a flake-parts configuration that passed ecosystemSrc threw. The selected source went back into resolveEcosystemSrc, which takes a function. flake-parts now resolves a selected source once. structuralTopMatchesFlakeParts covers it.

Tests: ecosystemsRecordWhereEachPartCameFrom and ecosystemsRecordWhereEachIntegrationsPartsCameFrom in the pinned-world suite, assertions in the existing test of each integration, and nixpkgs-lib-only for the package config, including a library that names no pinned source. The full nix flake check passes, VM tests included.

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