feat(kernel): emit ContextTruncatedEvent on workflow context overflow

DefaultContextPackBuilder already enforced the token budget, but SessionOrchestrator discarded the entriesDropped signal at both build sites — overflow was enforced yet unobserved. Emit ContextTruncatedEvent when entriesDropped>0, matching the router path (open-threads 6.2 contract).

testing/integration/src/test/kotlin/SessionOrchestratorIntegrationTest.kt

@@ -10,7 +10,13 @@ import com.correx.core.approvals.model.ApprovalDecision
 import com.correx.core.approvals.model.ApprovalScopeIdentity
 import com.correx.core.artifacts.DefaultArtifactReducer
 import com.correx.core.artifactstore.ArtifactStore
+import com.correx.core.context.builder.DefaultContextPackBuilder
+import com.correx.core.context.compression.CompressionStrategy
+import com.correx.core.context.compression.ContextCompressor
+import com.correx.core.context.model.ContextEntry
+import com.correx.core.context.model.TokenBudget
 import com.correx.core.events.events.ApprovalRequestedEvent
+import com.correx.core.events.events.ContextTruncatedEvent
 import com.correx.core.events.events.InferenceCompletedEvent
 import com.correx.core.events.events.InferenceStartedEvent
 import com.correx.core.events.events.OrchestrationPausedEvent
@@ -22,6 +28,7 @@ import com.correx.core.events.execution.RetryPolicy
 import com.correx.core.events.types.ArtifactId
 import com.correx.core.events.types.SessionId
 import com.correx.core.events.types.StageId
+import com.correx.core.events.types.TransitionId
 import com.correx.core.inference.InferenceRepository
 import com.correx.core.inference.InferenceRouter
 import com.correx.core.inference.InferenceState
@@ -39,6 +46,7 @@ import com.correx.core.sessions.ApprovalMode
 import com.correx.core.sessions.DefaultSessionRepository
 import com.correx.core.sessions.projections.replay.DefaultEventReplayer
 import com.correx.core.sessions.projections.replay.EventReplayer
+import com.correx.core.transitions.evaluation.PromptResolver
 import com.correx.core.transitions.graph.StageConfig
 import com.correx.core.transitions.graph.TransitionEdge
 import com.correx.core.transitions.graph.WorkflowGraph
@@ -304,6 +312,48 @@ class SessionOrchestratorIntegrationTest {
         assertNotNull(failed.reason)
     }
 
+    @Test
+    fun `context budget overflow on the workflow path emits ContextTruncatedEvent`(): Unit = runBlocking {
+        // A compressor that drops everything forces the builder to report dropped entries; the
+        // orchestrator must surface that as a ContextTruncatedEvent (mirroring the router path).
+        val droppingBuilder = DefaultContextPackBuilder(
+            object : ContextCompressor {
+                override fun compress(
+                    entries: List<ContextEntry>,
+                    budget: TokenBudget,
+                    strategy: CompressionStrategy,
+                ): List<ContextEntry> = emptyList()
+            },
+        )
+        val truncGraph = WorkflowGraph(
+            id = "trunc-test",
+            stages = mapOf(
+                StageId("A") to StageConfig(metadata = mapOf("systemPrompt" to "you are a helpful assistant")),
+            ),
+            transitions = setOf(
+                TransitionEdge(TransitionId("t1"), StageId("A"), StageId("done"), condition = { true }),
+            ),
+            start = StageId("A"),
+        )
+        val truncOrchestrator = DefaultSessionOrchestrator(
+            repositories = repositories,
+            // PromptResolver { it } makes the systemPrompt metadata resolve to non-blank text,
+            // so there is a compressible entry for the dropping compressor to discard.
+            engines = engines.copy(contextPackBuilder = droppingBuilder, promptResolver = PromptResolver { it }),
+            retryCoordinator = retryCoordinator,
+            artifactStore = artifactStore,
+        )
+        val sessionId = SessionId("s-trunc")
+        val config = OrchestrationConfig(retryPolicy = RetryPolicy(maxAttempts = 1, backoffMs = 0))
+
+        truncOrchestrator.run(sessionId, truncGraph, config)
+
+        val truncated = eventStore.read(sessionId).mapNotNull { it.payload as? ContextTruncatedEvent }
+        assertTrue(truncated.isNotEmpty(), "expected a ContextTruncatedEvent when the budget drops entries")
+        assertEquals("A", truncated.first().turnId)
+        assertTrue(truncated.first().entriesDropped >= 1)
+    }
+
     @Test
     fun `artifactStore put is called for prompt and response and ids appear on inference events`(): Unit = runBlocking {
         val recordingStore = RecordingArtifactStore()