nativeTexture FFI lookup targets wrong DLL and renderTextureObjPtr getter missing in lib/src/rive_native_ffi.dart

Description
Two issues in rive_native_ffi.dart on Windows:

A) Wrong DLL for nativeTexture lookup.

_nativeTexture is looked up from DynamicLibraryHelper.nativeLib, which resolves to rive_native.dll (the prebuilt library). On Windows, nativeTexture is not exported from rive_native.dll, causing a runtime crash:

Invalid argument(s): Failed to lookup symbol 'nativeTexture': 
error code 127 (The specified procedure could not be found.)

B) Missing renderTextureObjPtr getter.

_NativeRenderTexture exposes nativeRendererPointer (a Pointer), but not its integer address. Method channels cannot transport Pointer, so external bridges need the raw int address.

Changes We Made
A) Platform-conditional DLL resolution:

import 'dart:io' as io;

final DynamicLibrary nativeLib = DynamicLibraryHelper.nativeLib;

// On Windows, nativeTexture is exported from rive_native_plugin.dll (the
// Flutter plugin wrapper), not from rive_native.dll (the prebuilt Rive lib).
final DynamicLibrary _pluginLib = io.Platform.isWindows
    ? DynamicLibrary.open('rive_native_plugin.dll')
    : nativeLib;

// Changed from: nativeLib.lookup(...)
final Pointer<Void> Function(Pointer<Void>) _nativeTexture = _pluginLib
    .lookup<NativeFunction<Pointer<Void> Function(Pointer<Void>)>>(
      'nativeTexture',
    )
    .asFunction();

B) Added getter to
_NativeRenderTexture
:

dart

@override
  Pointer<Void> get nativeRendererPointer => _rendererPtr;
  /// The raw address of the native renderer object, suitable for passing
  /// through method channels to native code.
  int get renderTextureObjPtr => _rendererPtr.address;

Impact
(A) crashes at FFI lookup on Windows
(B) forces consumers to use unsafe (renderTexture as dynamic) casts

Note: If/when Ticket -> [(https://github.com/rive-app/rive-flutter/issues/610]) resolves (adding nativeTexture to the prebuilt rive_native.dll), issue (A) becomes unnecessary.

Environment
Package: rive_native 0.1.2
Platform: Windows
File: lib/src/rive_native_ffi.dart

Start in lib/src/rive_native_ffi.dart by tracing _nativeTexture through DynamicLibraryHelper.nativeLib and inspecting _NativeRenderTexture. Verify the Windows lookup targets the plugin DLL while other platforms retain their existing library, and confirm the renderer pointer can be exposed as an integer address for method-channel consumers.