settings: Tts:EngineByKind needs a structured editor — dropdowns for kind and engine, not freeform text

[genwave-radio]

From docs/THOUGHTS.md.

Tts:EngineByKind is freeform text today, but both dimensions are closed sets: segment kinds are a fixed enum, engines are our supported TTS providers. Freeform means typos ship silently. Give it the Corrections-style rule editor (kind dropdown × engine dropdown), and fix the Corrections editor's staging/edit-loss traps (gh-#139/gh-#140) first so the pattern being copied is a good one.