feat(journal-inmemory): add :journal-inmemory module with InMemoryJournalStore

- Introduced a new `:journal-inmemory` module implementing `:journal-api` with an in-memory backend.
- Added `InMemoryJournalStore` for concurrent append, list, and clear operations using `Mutex` and `MutableList`.
- Use cases include tests, dev mode, embedded scenarios, and client-side in-process caching.
- Integrated the module into the project setup and documented usage in `:client/README.md`.
- Added comprehensive

journal-inmemory/build.gradle.kts

@@ -0,0 +1,30 @@
+plugins {
+    alias(libs.plugins.kotlin.multiplatform)
+}
+
+// KMP-реализация [MutableJournalStore] на `MutableList` + `Mutex` — для
+// тестов, dev-режима, embedded-сценариев (Android core, CLI, in-process кэш
+// в клиенте) и как образец для своей реализации.
+//
+// `list` фильтрует по `conversationId`+`createdAt>after` и сортирует
+// по `createdAt ASC`. Paging — поверх отфильтрованного списка.
+//
+// Зависимости: только `:journal-api`. Никакого I/O — pure in-memory.
+
+kotlin {
+    jvmToolchain(21)
+
+    jvm()
+    linuxX64()
+    mingwX64()
+
+    sourceSets {
+        commonMain.dependencies {
+            api(project(":journal-api"))
+        }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+            implementation(libs.kotlinx.coroutines.test)
+        }
+    }
+}