task zz: add Voices()/BuildAttribution() to Provider interface [OCP]

Extend audio.Provider with Voices() []string and BuildAttribution() string
so all provider-specific behaviour is encapsulated in the implementation
rather than scattered as switch-cases across callers.

Add package-level VoicesFor(name) and BuildAttributionFor(name, params)
for callers (processor, GUI) that need these before constructing a
Provider instance. Add AttributionParamsFrom(config, word, ...) so
callers can build AttributionParams from the flat Config without a manual
provider switch. Implement both new interface methods in OpenAIProvider
and GeminiProvider. Update all Provider mock/fake types in tests.

Migrate audioVoicesForProvider() and saveAudioAttribution() in both
processor.go and gui/generator.go to use the new package-level helpers,
replacing the 10+ duplicated switch blocks.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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