terra: read wrapper settings from terranix _meta

[oneingan]

This lets the terra blocktype consume optional wrapper/backing settings from terranix metadata, without changing the rendered Terraform JSON.

• New (optional) contract consumed from terranix: _meta.std.*
  • _meta.std.package: terraform/opentofu package (optionally already withPlugins)
  • _meta.std.providers: list of provider packages; when set we call package.withPlugins
  • _meta.std.modules: attrset of vendored module directories (symlinked into the staging dir)
  • _meta.std.terraformBackendGit.*: override the default terraform-backend-git wrapper; enable = false runs the CLI directly

Dependency

• Uses terranix _meta passthru support: core: add internal _meta passthru terranix/terranix#151
  • Without _meta support, terra falls back to current behaviour (terraform + terraform-backend-git with the blocktype repo).

Testing

• nix flake check -L

[oneingan]

Update: removed the immutable root module machinery for now and kept this PR focused/minimal.

• terra only consumes optional _meta.std.{package,providers,modules,terraformBackendGit.*}
• no hard asserts; defaults preserve current behaviour when _meta is absent