fix: P1 TUI UX batch — race root cause, diff content, per-session routerMessages, optimistic IDLE submit

P1-1: Replace check-then-act with computeIfAbsent for activeSessionJobs to
prevent double-launch. Register pendingApprovals[requestId] BEFORE emitting
ApprovalRequestedEvent to close the approve-before-register window.
P1-2: ToolCompleted with a diff now appends msg.diff to routerMessages
instead of the static "--- diff from $toolName ---" marker.
P1-3: Change routerMessages from List<String> to Map<String, List<String>>
keyed by sessionId.value. RootReducer appends to the correct session's list;
RouterPanel renders only routerMessages[selectedId].
P1-4: On IDLE chat submit, SessionsReducer creates an optimistic
SessionSummary(status="STARTING") and sets selectedId.
processSessionStartedMessage removes any STARTING session and inserts the
real one on echo. RootReducer sets sessionEntered=true for the optimistic
session.
Tests: LaunchRegistrationRaceTest, ApprovalRegisterBeforeEmitTest,
RootReducerTest (diff/per-session/isolation/optimistic-submit),
SessionsReducerTest (optimistic/reconciliation). All P0/P1 tests pass.
./gradlew check green.

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -407,6 +407,8 @@ abstract class SessionOrchestrator(
                     }
                     // grant auto-approved — fall through to execute
                 } else {
+                    val deferred = CompletableDeferred<ApprovalDecision>()
+                    pendingApprovals[requestId] = deferred
                     emit(sessionId, OrchestrationPausedEvent(sessionId, stageId, "APPROVAL_PENDING"))
                     emit(
                         sessionId,
@@ -422,8 +424,6 @@ abstract class SessionOrchestrator(
                             preview = toolPreview ?: toolCall.function.arguments.take(200),
                         ),
                     )
-                    val deferred = CompletableDeferred<ApprovalDecision>()
-                    pendingApprovals[requestId] = deferred
                     val userDecision = try {
                         deferred.await()
                     } finally {
@@ -830,9 +830,11 @@ abstract class SessionOrchestrator(
         outcome: ValidationOutcome.NeedsApproval,
     ): StageExecutionResult {
         log.debug("[Orchestrator] approval required session={} stage={}", sessionId.value, stageId.value)
-        emit(sessionId, OrchestrationPausedEvent(sessionId, stageId, "APPROVAL_PENDING"))
         val domainRequest = buildApprovalRequest(sessionId, stageId, outcome)
+        val deferred = CompletableDeferred<ApprovalDecision>()
+        pendingApprovals[domainRequest.id] = deferred
 
+        emit(sessionId, OrchestrationPausedEvent(sessionId, stageId, "APPROVAL_PENDING"))
         emit(
             sessionId,
             ApprovalRequestedEvent(
@@ -846,9 +848,6 @@ abstract class SessionOrchestrator(
             ),
         )
 
-        val deferred = CompletableDeferred<ApprovalDecision>()
-        pendingApprovals[domainRequest.id] = deferred
-
         return try {
             log.debug(
                 "[Orchestrator] awaiting approval session={} requestId={}",