feat(persistence): P6a — use case RecordTurn (checkpoint fin de tour, isolé)

Logique du checkpoint conversationnel isolée et testable, sans câblage live :
append du tour au log canonique → load handoff → fold incrémental (tour neuf
seul) → save. Mapping StoreError→AppError via From existant. Debounce repoussé.

- application/src/conversation/{mod,record}.rs : RecordTurn::record
- tests : 6 cas (incrémentalité prouvée via summarizer-espion, propagation
  d'erreur sans effet de bord, isolation par conversation) ; suite verte

Câblage sur les seams réels (reply/ask) + composition root = P6b.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/application/src/conversation/mod.rs

@@ -0,0 +1,18 @@
+//! Conversation persistence use cases (cadrage « persistance conversationnelle »,
+//! ARCHITECTURE §19, lot P6).
+//!
+//! Where [`crate::agent::lifecycle`] owns the *runtime* of an agent (spawn, PTY,
+//! sessions), this module owns the **durable conversational memory** of a
+//! conversation (paire) : the canonical append-only log and its incremental
+//! handoff. The single use case here, [`RecordTurn`], materialises the
+//! **end-of-turn checkpoint** (D19-5) by talking **only** to the three driven/driving
+//! ports of §19 ([`domain::ConversationLog`], [`domain::HandoffStore`],
+//! [`domain::HandoffSummarizer`]) — never to a concrete adapter.
+//!
+//! This is the **isolated logic** of P6 (lot P6a) : no wiring to the orchestrator,
+//! the PTY or `app-tauri` (that is P6b). It is fully testable with in-memory fakes
+//! of the three ports.
+
+mod record;
+
+pub use record::RecordTurn;