server: convert :server from kotlin-jvm to KMP with all 9 targets

Punkt 6 of NATIVE-COMPATIBILITY.md. Mirrors :proto's target set:
jvm + macosX64 + macosArm64 + iosX64 + iosArm64 + iosSimulatorArm64
+ linuxX64 + linuxArm64 + mingwX64.

All 9 targets compile via './gradlew :server:assemble'.

No blockers: the only deps (ktor-server-core, ktor-server-content-
negotiation, ktor-serialization-kotlinx-json, kotlinx-coroutines-core,
kotlinx-serialization-json) ship native variants for every target.
kotlin.time.Instant was already in use (not the deprecated
kotlinx.datetime.Instant), so no proto-side work needed.

Only API change vs the JVM version: respondTextWriter (JVM-only) ->
respondBytesWriter + writeStringUtf8 (KMP, ByteWriteChannel API).
SSE behaviour is identical ('data: <json>\n\n' chunks flushed as
they arrive).

Verification: :standalone:jvmTest 44/44 green after the migration
(ChatAgent 15, LlmConfig 6, McpConfig 8, McpRegistry 4, Persistence 11).
No call-site changes needed; :standalone picks up the new :server
jvm artifact automatically.

server/build.gradle.kts

@@ -1,23 +1,41 @@
-import org.jetbrains.kotlin.gradle.dsl.JvmTarget
-
 plugins {
-    alias(libs.plugins.kotlin.jvm)
+    alias(libs.plugins.kotlin.multiplatform)
     alias(libs.plugins.kotlin.serialization)
 }
 
 kotlin {
-    compilerOptions {
-        jvmTarget.set(JvmTarget.JVM_21)
+    jvmToolchain(21)
+
+    // "Все возможные цели сборки": jvm + весь натив. Зеркалит набор :proto.
+    // Никаких блокеров нет: ktor-server-{core,sse,cio,content-negotiation} имеет
+    // опубликованные linuxX64/macosX64/macosArm64/iosX64/iosArm64/iosSimulatorArm64/
+    // linuxArm64/mingwX64 варианты; :proto уже собирается под все эти таргеты.
+    jvm()
+    macosX64()
+    macosArm64()
+    iosX64()
+    iosArm64()
+    iosSimulatorArm64()
+    linuxX64()
+    linuxArm64()
+    mingwX64()
+
+    sourceSets {
+        commonMain.dependencies {
+            implementation(project(":proto"))
+
+            // Ktor (без engine — engine подключает потребитель, см. :standalone).
+            implementation(libs.ktor.server.core)
+            implementation(libs.ktor.server.content.negotiation)
+            implementation(libs.ktor.serialization.kotlinx.json)
+
+            // Commons
+            implementation(libs.kotlinx.coroutines.core)
+            implementation(libs.kotlinx.serialization.json)
+        }
+        commonTest.dependencies {
+            implementation(libs.kotlinx.coroutines.core)
+            implementation(kotlin("test"))
+        }
     }
 }
-
-dependencies {
-    implementation(project(":proto"))
-
-    implementation(libs.ktor.server.core)
-    implementation(libs.ktor.server.content.negotiation)
-    implementation(libs.ktor.serialization.kotlinx.json)
-
-    implementation(libs.kotlinx.coroutines.core)
-    implementation(libs.kotlinx.serialization.json)
-}