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.

testing/deterministic/src/test/kotlin/L3MetadataRehydratorTest.kt

@@ -30,6 +30,7 @@ class L3MetadataRehydratorTest {
         }
         override suspend fun store(entry: L3MemoryEntry) = Unit
         override suspend fun query(query: L3Query): List<L3Hit> = emptyList()
+        override suspend fun existsByTurnIdPrefix(prefix: String): Boolean = false
         override suspend fun close() = Unit
     }
 

testing/deterministic/src/test/kotlin/RouterNarrationTest.kt

@@ -374,6 +374,8 @@ class RouterNarrationTest {
 
         override suspend fun query(query: L3Query): List<com.correx.core.router.l3.L3Hit> = emptyList()
 
+        override suspend fun existsByTurnIdPrefix(prefix: String): Boolean = false
+
         override suspend fun close() = Unit
     }
 }