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/src/commonMain/kotlin/pw/binom/agentik/server/Routes.kt

@@ -6,13 +6,14 @@ import io.ktor.server.application.ApplicationCall
 import io.ktor.server.application.call
 import io.ktor.server.request.receive
 import io.ktor.server.response.respond
+import io.ktor.server.response.respondBytesWriter
 import io.ktor.server.response.respondText
-import io.ktor.server.response.respondTextWriter
 import io.ktor.server.routing.Route
 import io.ktor.server.routing.delete
 import io.ktor.server.routing.get
 import io.ktor.server.routing.patch
 import io.ktor.server.routing.post
+import io.ktor.utils.io.writeStringUtf8
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.catch
 import kotlinx.serialization.KSerializer
@@ -147,13 +148,13 @@ private suspend fun <T> ApplicationCall.streamJsonSse(
     serializer: KSerializer<T>,
     json: Json = agentikJson,
 ) {
-    respondTextWriter(contentType = ContentType.Text.EventStream) {
+    respondBytesWriter(contentType = ContentType.Text.EventStream) {
         flow.catch { /* клиент отвалился — глушим */ }
             .collect { value ->
                 val s = json.encodeToString(serializer, value)
-                write("data: ")
-                write(s)
-                write("\n\n")
+                writeStringUtf8("data: ")
+                writeStringUtf8(s)
+                writeStringUtf8("\n\n")
                 flush()
             }
     }