[clhodapp]

Adds caisson.treefmt, the first of the survey's strong candidates, and the publishing mode it needs. This PR is based on #97. Merge #94 through #97 first, then change the base of this PR to main.

The treefmt integration

A treefmt configuration is the formatters of a tree, written as treefmt-nix modules. It is evaluated per system, once for each system in force, on the selected package set. The integration is named treefmt and resolves the ecosystem source treefmt-nix, as the darwin integration resolves nix-darwin.

treefmt-nix's own evalModule runs on the library of a nixpkgs. Instead, caisson evaluates treefmt-nix's published submodule-modules with its own evalModules on the composed library, with pkgs set to the selected set. Those modules need treefmt-nix's special arguments, which treefmt-nix publishes only inside the type its submoduleWith returns. caisson reads them from that type's functor.payload.specialArgs, an attribute of the module system that isn't documented as public.

The outputs are:

• wrapper, the program nix fmt runs;
• configFile, the treefmt.toml;
• programs, the formatters by name;
• check, a function from a source tree to a derivation that fails when the tree is not formatted.

Only the configuration named default is published, in two outputs:

Output	Value
formatter.<system>	its wrapper, which nix fmt runs
checks.<system>.treefmt	its check over the root of the tree, root on mkLib, which nix flake check builds; it fails when the tree is not formatted

The check is published where mkLib is given a root. A tree may declare treefmt configurations under other names beside default. For this, exportsTo may be a list of exports, and an export may carry only, a function of the manifest that says whether the configuration is published in that output. exportsOf reads them in the integrations library.

The pinned-world test checks four things:

• formatter holds the exact derivation treefmt-nix's mkWrapper builds from the same module;
• with a root, checks.<system> holds only treefmt, the configuration's check over the root;
• without a root, no check is published;
• a second configuration, docs, is evaluated.

Built for real, the published check succeeds on a formatted tree and fails on an unformatted one, with the diff.

Publishing keyed by system

Flakes key packages, formatter and others by system first, and caisson's publishing produced only flat names. exportsTo.bySystem publishes each configuration under the system of its evaluation, as <attrset>.<system>.<name>, named by the usual rules over its path without the systems on it. Combined with single, the output holds the configuration named default at each system, as formatter.<system> does. An output that mixes entries keyed by system with entries that aren't is refused, with their paths. A unit test covers all three cases. nvf, nixvim and kubenix will publish their packages this way.

Checks

• nix flake check passes.
• Caisson's overhead in the eval-weight check is 3.85 MB, against 3.78 MB on feat(facter): add the caisson-facter and facter integrations #97. The ceiling is 4.28 MB.
• All 18 workspace consumers pass nix flake check --no-build with this branch in place of both caisson pins.

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