feat(llm): on-air voice trio — they/their, persona-voiced openings, occasional self-name mentions

[genwave-radio]

Three audible on-air fixes, one commit each:

🗣️ #151 — they/their for artists (c12a22c)

System-prompt scaffold now carries: "Refer to artists and bands as they/them/their unless the provided metadata explicitly states pronouns - never infer gender from a name." Rides both prompt variants; pinned by specs on each. Closes #151.

🎭 #152 — neutral boilerplate only without a persona (95d7211)

BuildSystemPrompt split into NeutralOpening / PersonaOpening + shared scaffold: persona-voiced prompts drop "personality-neutral" entirely and open with "write every word in the voice of the persona described below." The contradiction Dean quoted from the demo box is gone. The issue's expose-the-prompt-in-Admin-UI half stays open, as scoped. (Refs #152 — not closed.)

📻 #150 — DJs occasionally say their own name (1492e13)

On ~15% of persona-voiced breaks (SelfNameMentionProbability const, "roughly one break in seven"), the persona section gains: "Name note: your on-air name is {name} - briefly work your own name into this break where it lands naturally… once is plenty." The roll lives at the call site (Random.Shared, same injected-nondeterminism posture as the TimeProvider clock) so the builder stays pure; persona gate enforced in the builder — no persona, no line, ever. Closes #150.

✅ dotnet build 0 warnings · Tts.Tests 243 passed · full solution suite 2,594 tests, 0 failures.