fix warnings

Author: majk-p

curl/src/main/scala/org/http4s/curl/internal/CurlEasy.scala

@@ -28,8 +28,12 @@ import scala.scalanative.unsigned._
 final private[curl] class CurlEasy private (val curl: Ptr[CURL], errBuffer: Ptr[CChar]) {
 
   // GC rooting for CFuncPtr wrappers — prevents Scala Native GC from collecting them
+  // These are intentionally write-only: they keep callback references alive on the heap.
+  @annotation.nowarn("msg=(never used|unused)")
   private var _headerCallback: Any = null
+  @annotation.nowarn("msg=(never used|unused)")
   private var _writeCallback: Any = null
+  @annotation.nowarn("msg=(never used|unused)")
   private var _readCallback: Any = null
 
   @inline private def throwOnError(thunk: => CURLcode): Unit = {

curl/src/main/scala/org/http4s/curl/unsafe/CurlExecutorScheduler.scala

@@ -40,8 +40,8 @@ final class CurlExecutorScheduler(
   private[this] var needsReschedule: Boolean = true
   private[this] val executeQueue: ArrayDeque[Runnable] = new ArrayDeque
   private[this] val sleepQueue: PriorityQueue[SleepTask] = new PriorityQueue
-  private[this] val callbacks
-      : mutable.Map[Ptr[libcurl.CURL], Either[Throwable, Unit] => Unit] = mutable.Map.empty
+  private[this] val callbacks: mutable.Map[Ptr[libcurl.CURL], Either[Throwable, Unit] => Unit] =
+    mutable.Map.empty
   private[this] val noop: Runnable = () => ()
 
   // ExecutionContext
@@ -184,7 +184,7 @@ final class CurlExecutorScheduler(
       IO(callbacks.remove(handle).foreach(_(Right(()))))
     }
 
-  private[this] final class SleepTask(val at: Long, val runnable: Runnable)
+  final private[this] class SleepTask(val at: Long, val runnable: Runnable)
       extends Runnable
       with Comparable[SleepTask] {
     def run(): Unit = { sleepQueue.remove(this); () }

curl/src/main/scala/org/http4s/curl/unsafe/CurlRuntime.scala

@@ -53,7 +53,7 @@ object CurlRuntime {
     if (_global == null) {
       installGlobal {
         CurlRuntime()
-      }
+      }: Unit
     }
 
     _global

tests/http/src/test/scala/CurlClientSuite.scala

@@ -21,7 +21,6 @@ import cats.effect.SyncIO
 import cats.effect.kernel.Resource
 import cats.effect.std.Random
 import cats.effect.unsafe.IORuntime
-import cats.syntax.all._
 import munit.CatsEffectSuite
 import org.http4s.Method._
 import org.http4s.Request