Add native Dang checks for SDK helpers

[shykes]

Add a workspace-installed sdk-helpers-dev module with 103 separate Dang @check functions. Run the suite with dagger check from this repository.

The checks cover the 93 saved comparison cases and 10 regression cases. They compare exact TOML and JSON bytes, file names, and mode 0644. They also verify all runtimes, loading and updates, validation, version handling, the public API, independent builder branches, and dependency ordering. Expected results and input files are committed with the checks.

Improve load errors to identify invalid fields and their expected types. Use stable merge sort with cached dependency keys. Align source argument spelling with the existing loadToml and loadJson public names. Add usage and test instructions.

Validation: dagger check passes all 103 checks with Dagger v1.0.0-beta.11. dagger functions confirms that the root exposes only moduleManifest, and the builder exposes the required 19 functions. The review found no production blocker in these changes.