fix: complete all P2 audit findings — dead code, NPE guard, hygiene

P2-1: Guard SessionId NPE with safe-call in SessionsReducer
P2-2: Remove 16 dead event classes + reducer branches; replace with live orchestration events in tests
P2-3: Standardize ChatInput divergences — guard CancelSession, single-arg ProtocolError
P2-4: Remove dead TuiToolRecord.diff and ToolDisplayStatus.REQUESTED
P2-5: Remove dead streamLive from SessionEventBridge
P2-6: Extract SessionsReducerContext data class for 6+ param method
P2-7: Replace StageId("none") sentinel with TypeId.NONE
P2-9: Add TypeId.random() factory on all type-alias IDs
P2-10: Add KDoc on schemaVersion documenting reserved-for-migration
P2-8: Verified RouterReducer string-template already correct (TypeId value class toString)

core/router/src/main/kotlin/com/correx/core/router/RouterContextBuilder.kt

@@ -103,7 +103,7 @@ class DefaultRouterContextBuilder(
         return ContextPack(
             id = ContextPackId("${state.sessionId?.value ?: "unknown"}-router-pack"),
             sessionId = state.sessionId ?: SessionId("unknown"),
-            stageId = state.currentStageId ?: StageId("none"),
+            stageId = state.currentStageId ?: StageId.NONE,
             layers = layers,
             budgetUsed = budgetUsed,
             budgetLimit = budget.limit,

core/router/src/main/kotlin/com/correx/core/router/RouterFacade.kt

@@ -46,7 +46,7 @@ class DefaultRouterFacade(
         history.add(RouterTurn(role = TurnRole.USER, content = input, timestamp = Clock.System.now()))
 
         val stateWithHistory = state.copy(conversationHistory = history.toList())
-        val effectiveStageId = state.currentStageId ?: StageId("none")
+        val effectiveStageId = state.currentStageId ?: StageId.NONE
 
         val contextPack = routerContextBuilder.build(stateWithHistory, config.tokenBudget)
         val provider = inferenceRouter.route(effectiveStageId, setOf(ModelCapability.General))