feat(domain,infra,app): rotation/rétention log.jsonl + lecture paginée (LS6)

Backend uniquement (UI React repoussée à LS7) :
- domain : port ConversationArchive + structs SegmentStats/PageCursor/PageDirection/
  TurnSlice/RotationDecision/RotationThresholds, fn pures rotation_plan/clamp_page_limit
  + consts ; re-exports lib.
- infrastructure : impl ConversationArchive pour FsConversationLog (stats/rotate/page
  + helpers), archive segmentée hors chemin chaud.
- application : ReadConversationPage + DTO + ConversationArchiveProvider (conversation/paginate),
  RotateConversationLog (conversation/rotate), exports mod/lib.
- app-tauri : AppConversationArchiveProvider + wiring (state), rotation détachée dans
  launch_agent + commande read_conversation_page (commands), DTOs (dto),
  commande enregistrée (generate_handler!).
- tests (QA, verts) : conversation_log, conversation_rotate_paginate (nouveau),
  dto (module test). Pivots INV-LS6 et cohérence fold-après-rotation verts.

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

crates/application/src/conversation/mod.rs

@@ -13,6 +13,13 @@
 //! the PTY or `app-tauri` (that is P6b). It is fully testable with in-memory fakes
 //! of the three ports.
 
+mod paginate;
 mod record;
+mod rotate;
 
+pub use paginate::{
+    ConversationArchiveProvider, ReadConversationPage, ReadConversationPageInput, TurnPage,
+    TurnSource, TurnView,
+};
 pub use record::RecordTurn;
+pub use rotate::{RotateConversationLog, RotateConversationLogInput};