Non-mutating extensions that help Mutare understand source code.

An extension implements one or both of these behaviours:

• Mutare.CallRouting — static or shape-aware macro-argument routing;
• Mutare.UseExpansion — an override for a use that cannot be expanded normally.

Extensions do not produce mutations or appear in reports. Add them to :extensions as modules or {module, opts} pairs:

[extensions: [Mutare.Gettext]]

Entries may be bare modules or {module, opts} pairs. Options are delivered only to Mutare.UseExpansion.expand_use/3; Mutare.CallRouting.call_routes/0 declarations and Mutare.CallRouting.route_arguments/2 classification are intentionally options-independent.

Mutators may implement Mutare.CallRouting too, but belong under :mutators. They are rejected from :extensions so their mutation producers cannot be enabled accidentally as routing-only modules.

An extension that routes a library's DSL may declare that library's modules by exporting required_modules/0 (the same optional callback mutators declare — see Mutare.Mutator.required_modules/0); validate!/1 checks each is loadable and aborts with a Mutare.EnvironmentError otherwise, so an external-source run fails loudly at startup instead of silently registering routes against nothing.