feat(sidecar): scaffold Kotlin JVM module with Shadow fat JAR

Kotlin 2.1.21 (deviation from plan's 1.9, forced by Gradle 9
compatibility), JDK 17 toolchain, application plugin with main
entry dev.uatu.sidecar.MainKt. Shadow 9.0.0-rc2 for the fat JAR.
Actual gRPC server wires in with task #10.

sidecar/src/main/kotlin/dev/uatu/sidecar/Main.kt

@@ -0,0 +1,5 @@
+package dev.uatu.sidecar
+
+fun main() {
+    println("uatu-sidecar stub: gRPC server lands in task #10")
+}

sidecar/src/test/kotlin/dev/uatu/sidecar/MainTest.kt

@@ -0,0 +1,11 @@
+package dev.uatu.sidecar
+
+import kotlin.test.Test
+import kotlin.test.assertTrue
+
+class MainTest {
+    @Test
+    fun mainExists() {
+        assertTrue(true, "placeholder test — real tests land with DriverService")
+    }
+}