feat(router,inference,config): pin narration to a configured model_id

[router.narration] model_id selects which provider handles narration turns
instead of generic capability routing — lets a small/fast model own narration
while the main model keeps CHAT/STEERING.

- NarrationSettings.modelId parsed from TOML, threaded via RouterConfig
  .narrationModelId into DefaultRouterFacade.narrate (3-arg route)
- DefaultInferenceRouter: exact-id match against healthy providers, with a
  post-select health check; any miss logs WARN and falls back to capability
  routing (never fails the narration turn)
- onUserInput unchanged — only narrate uses the pinned model (tested)

core/config/src/main/kotlin/com/correx/core/config/ConfigLoader.kt

@@ -494,6 +494,7 @@ object ConfigLoader {
             topP = asDouble(routerNarrationSection["top_p"], 0.9),
             maxTokens = asInt(routerNarrationSection["max_tokens"], 4096),
             maxPerRun = asInt(routerNarrationSection["max_per_run"], 100),
+            modelId = asStringOrNull(routerNarrationSection["model_id"]),
         )
 
         val router = RouterConfig(

core/config/src/main/kotlin/com/correx/core/config/CorrexConfig.kt

@@ -157,6 +157,7 @@ data class NarrationSettings(
     val topP: Double = 0.9,
     val maxTokens: Int = 4096,
     val maxPerRun: Int = 100,
+    val modelId: String? = null,
 )
 
 @Serializable

core/config/src/test/kotlin/com/correx/core/config/ConfigLoaderTest.kt

@@ -2,6 +2,7 @@ package com.correx.core.config
 
 import org.junit.jupiter.api.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertNull
 
 class ConfigLoaderTest {
     @Test
@@ -519,4 +520,28 @@ class ConfigLoaderTest {
         assertEquals(180_000L, result.orchestration.stageTimeoutMs)
         assertEquals(2_000, result.orchestration.journalCompactionTokenThreshold)
     }
+
+    @Test
+    fun `parseToml parses router narration model_id`() {
+        val toml = """
+            [router.narration]
+            model_id = "llama-cpp:phi-3-mini"
+        """.trimIndent()
+
+        val result = ConfigLoader.parseTomlForTest(toml)
+
+        assertEquals("llama-cpp:phi-3-mini", result.router.narration.modelId)
+    }
+
+    @Test
+    fun `parseToml narration model_id defaults to null when absent`() {
+        val toml = """
+            [router.narration]
+            max_tokens = 2048
+        """.trimIndent()
+
+        val result = ConfigLoader.parseTomlForTest(toml)
+
+        assertNull(result.router.narration.modelId)
+    }
 }