feat(memory): state-keyed repo-map reuse via L3 presence check; embed entries on fresh scan

- Add existsByTurnIdPrefix(prefix) to L3MemoryStore interface; implemented in
  InMemoryL3MemoryStore (mutex-guarded) and TurboVecL3MemoryStore (metadata map).
  All test fakes (TrackingL3MemoryStore, CapturingStore) updated.
- Introduce RepoMapIndexerPort interface; RepoMapIndexer implements it. Allows
  injection of test doubles without framework overhead.
- ProjectMemoryService.indexAndRecord: reads latest WorkspaceStateObservedEvent
  from event store (invariant #9 — recorded fact, not re-probe); skips scan+embed
  when L3 already holds entries for "repomap:<root>:<stateKey>"; passes stateKey
  to RepoMapComputedEvent; embeds each scanned entry after append; per-entry
  failures warn-logged (CancellationException rethrown).
- Tests: InMemoryL3MemoryStoreTest +2 cases; new ProjectMemoryServiceReuseTest
  covers same-key skip, changed-key re-index, no-observation always-reindex,
  and embedding turnId tag verification.

core/router/src/main/kotlin/com/correx/core/router/l3/InMemoryL3MemoryStore.kt

@@ -33,6 +33,9 @@ class InMemoryL3MemoryStore : L3MemoryStore {
         }
     }
 
+    override suspend fun existsByTurnIdPrefix(prefix: String): Boolean =
+        mutex.withLock { entries.any { it.turnId.startsWith(prefix) } }
+
     override suspend fun close() {
         // No-op for in-memory store
     }

core/router/src/main/kotlin/com/correx/core/router/l3/L3MemoryStore.kt

@@ -15,6 +15,11 @@ interface L3MemoryStore {
      */
     suspend fun query(query: L3Query): List<L3Hit>
 
+    /**
+     * Returns true if any stored entry has a [L3MemoryEntry.turnId] that starts with [prefix].
+     */
+    suspend fun existsByTurnIdPrefix(prefix: String): Boolean
+
     /**
      * Close the store and release resources.
      */

core/router/src/test/kotlin/com/correx/core/router/l3/InMemoryL3MemoryStoreTest.kt

@@ -51,6 +51,33 @@ class InMemoryL3MemoryStoreTest {
         assertEquals("entry-2", hits[1].entry.id, "Second match should be entry-2")
     }
 
+    @Test
+    fun `existsByTurnIdPrefix returns true when entry turnId starts with prefix`(): Unit = runBlocking {
+        val store = InMemoryL3MemoryStore()
+        val sessionId = TypeId("session-1")
+        store.store(
+            L3MemoryEntry(
+                id = "e1", sessionId = sessionId, turnId = "repomap:/repo:git:abc123",
+                text = "foo", vector = floatArrayOf(1f), timestampMs = 1L
+            )
+        )
+        assert(store.existsByTurnIdPrefix("repomap:/repo:git:abc123")) { "should match exact" }
+        assert(store.existsByTurnIdPrefix("repomap:/repo:git:")) { "should match prefix" }
+    }
+
+    @Test
+    fun `existsByTurnIdPrefix returns false when no entry matches`(): Unit = runBlocking {
+        val store = InMemoryL3MemoryStore()
+        val sessionId = TypeId("session-1")
+        store.store(
+            L3MemoryEntry(
+                id = "e1", sessionId = sessionId, turnId = "project:/repo",
+                text = "foo", vector = floatArrayOf(1f), timestampMs = 1L
+            )
+        )
+        assert(!store.existsByTurnIdPrefix("repomap:/repo:git:")) { "should not match different prefix" }
+    }
+
     @Test
     fun `respects sessionIdFilter`(): Unit = runBlocking {
         val store = InMemoryL3MemoryStore()