feat(persona,LaneD): response_prompt() lazy output on PersonaTurnFrame

[joelteply]

Summary

Lane D advancement. Adds the chat-style prompt lazy output the inference engine consumes. Closes the chain from inbox event → turn frame → ready-to-infer prompt, fully in Rust.

Pure Rust, zero TS.

What lands

• ResponsePrompt struct: persona_id, room_id, system_prompt (Option; caller fills from IdentityState), messages (Vec), trigger_message_id
• PromptRole enum: System / User / Assistant — chat-completion taxonomy
• PromptMessage { role, content } — one turn in the prompt
• PersonaTurnFrame::response_prompt() — fourth lazy output alongside consolidated_inbox + rag_seed + replay_record

Design

• Every inbox message becomes a User-role PromptMessage in chronological order. The persona's identity (System role) is filled in by the caller from IdentityState (not loaded into the turn frame today; future PR may add lazily).
• Per Joel's "Rust owns behavior" + "no TS shimming Rust outputs": the substrate owns the prompt-build path so TS PRG doesn't wrap a raw transcript into a model-specific prompt format.
• Wire shape: camelCase fields (systemPrompt, triggerMessageId) + lowercase role enum (system/user/assistant) — matches the de-facto chat-completion JSON.
• Returns None for empty frames (same contract as consolidated_inbox + rag_seed — empty inbox = no turn to plan).

This is the lazy output that PR-4 inference-llm's (#1395) InferenceRequest.prompt_text expects. A follow-up PR will add the turn-execute command chaining drain-turn-frame → response_prompt → inference/llm/request for the full persona-turn-in-one-Rust-call.

Test plan

• cargo test --lib --features metal,accelerate persona::turn_frame — 9/9 pass (5 new + 4 existing):
  • response_prompt_returns_none_for_empty_frame
  • response_prompt_carries_one_user_message_per_inbox_message
  • response_prompt_system_prompt_is_none_pr1 (pins the IdentityState separation)
  • response_prompt_trigger_matches_latest_message_id
  • response_prompt_round_trips_through_serde (wire stability)
• No regressions across other 2973 lib tests
• Pre-push gate clean

Stack

• Lane D PR-1/2/3 skeleton + drain_frame + rag_seed — already shipped
• feat(cognition,LaneD): persona/drain-turn-frame command — Rust-owned turn-frame wrap #1398 — Lane D drain-turn-frame command (mine, just merged)
• This PR — Lane D ResponsePrompt lazy output (the inference-input lazy node ALPHA-GAP named)
• NEXT — turn-execute command chaining drain → response_prompt → inference/llm/request

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