[clhodapp]

Builds on nix-caisson/caisson-core#51, merged, which gives every configuration manifest a history. This branch pins caisson-core to main, which has it.

caisson fills each configuration's history with a declaration event for each module its evaluator received, in that order. For the NixOS configuration of the pinned-world machine, the events are:

Key	Origin
nixos	nixpkgs: nixos/modules/module-list.nix (NixOS' module list, a single event)
caisson.nixos	caisson: modules/nixos/core
modules.nixos.caisson/core	project caisson
modules.nixos.caisson/default	project caisson
inline/4	the configuration's module
caisson.nixos.pkgSet	caisson's module that hands NixOS the package set

Keys. lib.caisson.modules now hands out each registered module inside a module keyed modules.<class>.<name>@<project>, so an evaluation knows which registry entry each module is. The module system imports modules that share a key once, so a registry entry an evaluation selects twice is imported once. Before, it was imported twice, which either failed or doubled its list definitions. The project is part of the key because the same name can be registered by different projects in the registries an evaluation reads. The tree has a local home-manager-activation in a unit test, and caisson's integrations add their own through caisson's closure. Without the project in the key, the module system would drop one of them silently.

Integrations. Each integration's evaluate returns modules, the list it handed its evaluator, with ecosystemUnit { key; project; file; } where the ecosystem's module list goes: NixOS, nix-darwin, home-manager, nvf, nixidy, system-manager, terranix, flake-parts and treefmt-nix. inEcosystem makes those units for hjem, nixvim and nix-topology, whose lists name the ecosystem's modules by their files. The other integrations evaluate only caisson's modules and the configuration's.

Files. A file inside a source in the Nix store is written relative to that source, such as modules/nixos/core. A module registered through projects has no file yet, because registration doesn't record one. caisson-core's own registry events have no file for modules either.

Tests.

• The pinned-world test aMachineRecordsWhatItsEvaluatorReceived checks the sequence above, its origins and indices, and that a home-manager configuration inside the machine has the machine's childless history as its prefix and adds events after it.
• Two unit tests that read lib.caisson.modules directly now look inside the keying module.

nix flake check passes locally, including the VM tests and the evaluation-cost gate.

The docs are in nix-caisson/nix-caisson.github.io#81.

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