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/inference/llama_cpp/build.gradle

@@ -22,6 +22,7 @@ dependencies {
     implementation "io.ktor:ktor-client-content-negotiation:$ktor_version"
     implementation "io.ktor:ktor-serialization-kotlinx-json:$ktor_version"
     testImplementation "org.junit.jupiter:junit-jupiter"
+    testImplementation "io.ktor:ktor-client-mock:$ktor_version"
 }
 
 tasks.named("koverVerify").configure { enabled = false }