Add missing @internal annotations

src/Internal/Posix/PosixRunner.php

@@ -10,6 +10,7 @@ use Amp\Process\Internal\ProcessStatus;
 use Amp\Process\ProcessException;
 use Revolt\EventLoop;
 
+/** @internal */
 final class PosixRunner implements ProcessRunner
 {
     private const FD_SPEC = [

src/Internal/ProcHolder.php

@@ -2,6 +2,7 @@
 
 namespace Amp\Process\Internal;
 
+/** @internal */
 final class ProcHolder
 {
     private ProcessRunner $runner;

src/Internal/ProcessHandle.php

@@ -6,6 +6,7 @@ use Amp\ByteStream\ReadableResourceStream;
 use Amp\ByteStream\WritableResourceStream;
 use Amp\DeferredFuture;
 
+/** @internal */
 abstract class ProcessHandle
 {
     /** @var resource */

src/Internal/ProcessRunner.php

@@ -4,6 +4,7 @@ namespace Amp\Process\Internal;
 
 use Amp\Process\ProcessException;
 
+/** @internal */
 interface ProcessRunner
 {
     /**

src/Internal/ProcessStatus.php

@@ -2,6 +2,7 @@
 
 namespace Amp\Process\Internal;
 
+/** @internal */
 final class ProcessStatus
 {
     public const STARTING = 0;