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/domain/tests/layout.rs

@@ -20,6 +20,7 @@ fn leaf(id: u128, sess: Option<u128>) -> LeafCell {
         session: sess.map(session),
         agent: None,
         conversation_id: None,
+        engine_session_id: None,
         agent_was_running: false,
     }
 }
@@ -32,6 +33,7 @@ fn leaf_with_resume(id: u128, sess: Option<u128>) -> LeafCell {
         session: sess.map(session),
         agent: None,
         conversation_id: Some("conv-1".to_string()),
+        engine_session_id: None,
         agent_was_running: true,
     }
 }
@@ -674,6 +676,7 @@ fn agent_leaves_collects_only_agent_bearing_leaves() {
                     session: None,
                     agent: Some(agent_id(100)),
                     conversation_id: None,
+                    engine_session_id: None,
                     agent_was_running: false,
                 }),
                 weight: 1.0,
@@ -688,6 +691,7 @@ fn agent_leaves_collects_only_agent_bearing_leaves() {
                     session: None,
                     agent: Some(agent_id(101)),
                     conversation_id: None,
+                    engine_session_id: None,
                     agent_was_running: false,
                 }),
                 weight: 1.0,
@@ -783,6 +787,7 @@ fn move_session_preserves_resume_fields_on_both_leaves() {
                     session: Some(session(100)),
                     agent: None,
                     conversation_id: Some("conv-from".to_string()),
+                    engine_session_id: None,
                     agent_was_running: true,
                 }),
                 weight: 1.0,
@@ -793,6 +798,7 @@ fn move_session_preserves_resume_fields_on_both_leaves() {
                     session: None,
                     agent: None,
                     conversation_id: Some("conv-to".to_string()),
+                    engine_session_id: None,
                     agent_was_running: true,
                 }),
                 weight: 1.0,
@@ -842,6 +848,7 @@ fn agent_leaf_full(
         session: None,
         agent: Some(agent_id(agent)),
         conversation_id: conv.map(str::to_string),
+        engine_session_id: None,
         agent_was_running: running,
     }
 }
@@ -1047,6 +1054,7 @@ fn leaf_serde_all_four_combinations_roundtrip() {
             session: None,
             agent: None,
             conversation_id: conv.clone(),
+            engine_session_id: None,
             agent_was_running: running,
         };
         let json = serde_json::to_string(&cell).unwrap();
@@ -1062,6 +1070,7 @@ fn leaf_serde_omits_defaults() {
         session: None,
         agent: None,
         conversation_id: None,
+        engine_session_id: None,
         agent_was_running: false,
     };
     let json = serde_json::to_string(&cell).unwrap();
@@ -1082,6 +1091,7 @@ fn leaf_serde_field_names_are_camel_case_when_present() {
         session: None,
         agent: None,
         conversation_id: Some("c".to_string()),
+        engine_session_id: None,
         agent_was_running: true,
     };
     let json = serde_json::to_string(&cell).unwrap();
@@ -1108,6 +1118,7 @@ fn leaf_can_carry_conversation_without_session_and_inversely() {
         session: None,
         agent: None,
         conversation_id: Some("c".to_string()),
+        engine_session_id: None,
         agent_was_running: false,
     };
     let a_back: LeafCell = serde_json::from_str(&serde_json::to_string(&a).unwrap()).unwrap();
@@ -1119,6 +1130,7 @@ fn leaf_can_carry_conversation_without_session_and_inversely() {
         session: Some(session(7)),
         agent: None,
         conversation_id: None,
+        engine_session_id: None,
         agent_was_running: false,
     };
     let b_back: LeafCell = serde_json::from_str(&serde_json::to_string(&b).unwrap()).unwrap();