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 }