feat: L3 memory retrieval on the router read path (record-only)

When a user sends input, embed it, query L3 across all sessions, dedup
against in-session turns, and record the retrieval as an event so replay
is deterministic (invariant #9). Hits land in RouterState; context
injection follows in a later slice.

- Add L3MemoryRetrievedEvent + L3RetrievedHit (registered in eventModule)
- RouterState.lastRetrievedMemory + reducer case; RouterTurn carries turnId
- RouterConfig.retrievalK (default 5)
- Harden L3 write path: runCatching + visible logging, cancellation
  re-thrown; event append stays ahead of the L3 write
- Warn prominently when the non-durable in_memory L3 backend is selected

infrastructure/build.gradle

@@ -27,6 +27,7 @@ dependencies {
     implementation "io.ktor:ktor-client-cio:$ktor_version"
     implementation "io.ktor:ktor-client-content-negotiation:$ktor_version"
     implementation "io.ktor:ktor-serialization-kotlinx-json:$ktor_version"
+    implementation "org.slf4j:slf4j-api:2.0.16"
 }
 
 tasks.named("koverVerify").configure { enabled = false }

infrastructure/src/main/kotlin/com/correx/infrastructure/InfrastructureModule.kt

@@ -51,12 +51,15 @@ import com.correx.infrastructure.workflow.PromptLoader
 import com.correx.infrastructure.workflow.TomlWorkflowLoader
 import com.correx.infrastructure.workflow.WorkflowLoader
 import kotlinx.coroutines.runBlocking
+import org.slf4j.LoggerFactory
 import java.nio.file.Files
 import java.nio.file.Path
 import java.nio.file.Paths
 import java.nio.file.StandardCopyOption
 import java.sql.DriverManager
 
+private val log = LoggerFactory.getLogger(InfrastructureModule::class.java)
+
 @Suppress("TooManyFunctions")
 object InfrastructureModule {
     private val defaultDbPath: String =
@@ -199,7 +202,14 @@ object InfrastructureModule {
 
     fun createL3MemoryStoreFromConfig(config: L3Config): L3MemoryStore {
         return when (config.backend) {
-            "in_memory" -> createInMemoryL3MemoryStore()
+            "in_memory" -> {
+                log.warn(
+                    "L3 backend is 'in_memory': non-durable, intended for tests/dev only — " +
+                        "all router memory is lost on restart. " +
+                        "Set [l3] backend = \"turbovec\" for persistence."
+                )
+                createInMemoryL3MemoryStore()
+            }
             "turbovec" -> createTurboVecL3MemoryStore(config)
             else -> throw IllegalArgumentException(
                 "Unknown L3 backend: '${config.backend}'. Supported: in_memory, turbovec"