feat: switchable router embedder and L3 backends via config

Adds [router.embedder] and [router.l3] sections to CorrexConfig with
backend selectors. Ships LlamaCppEmbedder that hits llama.cpp's
/embedding endpoint (handles OpenAI-compatible, simple, and array
response shapes; validates dimension). InfrastructureModule gains
createEmbedderFromConfig and createL3MemoryStoreFromConfig that
dispatch on backend value.

Defaults preserve current behavior (noop embedder + in-memory L3).
Switching to "llamacpp" / "turbovec" is a config-only change — no code
edits required. For turbovec backend, the bundled python sidecar
script is extracted from classpath to ~/.cache/correx/ on first use.

infrastructure/build.gradle

@@ -11,10 +11,12 @@ dependencies {
     implementation project(":core:inference")
     implementation project(":core:approvals")
     implementation project(":core:sessions")
+    implementation project(":core:config")
     implementation project(":infrastructure:inference")
     implementation project(":infrastructure:inference:commons")
     implementation project(":infrastructure:inference:llama_cpp")
     implementation project(":infrastructure:persistence")
+    implementation project(":infrastructure:router:turbovec")
     implementation project(":infrastructure:tools")
     implementation project(":infrastructure:tools:filesystem")
     implementation project(":infrastructure:workflow")