feat(persistence): P8a — cohérence de clé conversation (id de paire stable)

Corrige l'incohérence P6/P7 : la cellule et la persistance partagent désormais
le MÊME id de paire IdeA, déterministe et stable au redémarrage — la clé sous
laquelle P6b sauve log/handoff == celle sous laquelle P7 les charge.

- domaine : ConversationId::for_pair(a,b) pur/déterministe (User↔Agent = uuid
  agent ; Agent↔Agent = XOR commutatif) ; LeafCell gagne engine_session_id
  (cache resumable moteur, additif serde default)
- infrastructure : InMemoryConversationRegistry::resolve utilise for_pair au
  lieu de new_random() → id recalculable sans état, identique après restart
- application : launch_structured persiste l'id de paire sur la cellule (et l'id
  moteur sur engine_session_id) ; cellule neuve ⇒ dérive for_pair(User,agent) ;
  resolve_conversation repli factorisé sur for_pair ; withers layout clone-and-
  mutate (préservent les champs additifs)
- app-tauri : engine_session_id propagé dans les DTO

Suites domain/infra/application/app-tauri vertes (assertion structured_launch_d3
recodée sur le contrat). Test déterminisme/restart dédié à ajouter au retour de
quota agent (binôme Test interrompu par limite de session).

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

crates/app-tauri/tests/dto.rs

@@ -308,6 +308,7 @@ fn layout_dto_serialises_camelcase_tagged_tree() {
         session: None,
         agent: None,
         conversation_id: None,
+        engine_session_id: None,
         agent_was_running: false,
     });
     let dto = LayoutDto::from(LoadLayoutOutput {
@@ -409,6 +410,7 @@ fn layout_dto_round_trips_a_split_tree_shape() {
         session: None,
         agent: None,
         conversation_id: None,
+        engine_session_id: None,
         agent_was_running: false,
     })
     .split(
@@ -419,6 +421,7 @@ fn layout_dto_round_trips_a_split_tree_shape() {
             session: None,
             agent: None,
             conversation_id: None,
+            engine_session_id: None,
             agent_was_running: false,
         },
         nid(9),

crates/app-tauri/tests/dto_agents.rs

@@ -307,6 +307,7 @@ fn launch_agent_output_maps_to_terminal_session_dto() {
     let out = LaunchAgentOutput {
         session,
         assigned_conversation_id: None,
+        engine_session_id: None,
         structured: None,
     };
     let dto = TerminalSessionDto::from(out);
@@ -351,6 +352,7 @@ fn launch_agent_output_propagates_assigned_conversation_id() {
     let out = LaunchAgentOutput {
         session,
         assigned_conversation_id: Some("conv-xyz".to_owned()),
+        engine_session_id: None,
         structured: None,
     };
     let dto = TerminalSessionDto::from(out);

crates/app-tauri/tests/dto_chat.rs

@@ -149,6 +149,7 @@ fn launch_output_with_structured_descriptor_derives_chat_cell_kind() {
     let out = LaunchAgentOutput {
         session,
         assigned_conversation_id: None,
+        engine_session_id: None,
         structured: Some(descriptor),
     };
     let dto = TerminalSessionDto::from(out);
@@ -164,6 +165,7 @@ fn launch_output_without_structured_descriptor_derives_pty_cell_kind() {
     let out = LaunchAgentOutput {
         session,
         assigned_conversation_id: None,
+        engine_session_id: None,
         structured: None,
     };
     let dto = TerminalSessionDto::from(out);
@@ -197,6 +199,7 @@ fn pty_launch_output_serialises_cellkind_pty_without_breaking_existing_keys() {
     let out = LaunchAgentOutput {
         session,
         assigned_conversation_id: None,
+        engine_session_id: None,
         structured: None,
     };
     let v = serde_json::to_value(TerminalSessionDto::from(out)).unwrap();