internal/tts becomes a thin backward-compat alias over internal/audio:
15 type aliases, 6 constants, 5 constructors, 5 compile-time signature
guards. All pre-refactor callers compile unchanged. alias_test.go
enforces symbol coverage and type identity. Old per-provider files
(manager, types, elevenlabs, openai, edge, minimax) are removed in
the same commit to keep history bisectable.
