Non-mutating extensions for call routing, use expansion, and coverage attribution.

An extension implements one or more of these behaviours:

• Mutare.CallRouting — static or shape-aware macro-argument routing;
• Mutare.UseExpansion — an override for a use that cannot be expanded normally;
• Mutare.CoverageAttribution — hooks, installed in the coverage probe's test VM, that tell Mutare which test a process with no lineage link to one is working for.

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 to Mutare.UseExpansion.expand_use/3 and Mutare.CoverageAttribution.attach_attribution/1; Mutare.CallRouting.call_routes/0 declarations and Mutare.CallRouting.route_arguments/1 classification are intentionally options-independent. The probe's test VM receives attach_attribution/1's options as written-out source, so an extension exporting it must be given plain data (atoms, numbers, strings, and lists, tuples and maps of them); anything else is refused here.

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.