refactor: rearrange some stubs/imports for easier web WASM deployment

thermion_dart/lib/thermion_dart/compatibility/compatibility.dart

@@ -1,3 +1 @@
-export 'web/compatibility.dart' if (dart.library.io) 'native/compatibility.dart';
-
-
+export 'native/compatibility.dart';

thermion_dart/lib/thermion_dart/compatibility/web/ffi/allocator.dart

@@ -6,7 +6,7 @@ import 'dart:convert';
 import 'dart:ffi' as ffi hide Uint8Pointer, FloatPointer;
 import 'dart:typed_data';
 
-import 'package:thermion_dart/thermion_dart/compatibility/web/thermion_dart.g.dart';
+import 'package:thermion_dart/thermion_dart/compatibility/web/ffi/thermion_dart.g.dart';
 
 import 'package:ffi/ffi.dart';
 export 'package:ffi/ffi.dart' hide StringUtf8Pointer, Utf8Pointer;

thermion_dart/lib/thermion_dart/compatibility/web/ffi/compatibility_ffi.dart

@@ -1,6 +1,6 @@
 import 'dart:async';
 import 'dart:js_interop';
-import 'package:thermion_dart/thermion_dart/compatibility/web/interop.dart';
+import 'package:thermion_dart/thermion_dart/compatibility/web/ffi/interop.dart';
 
 import "allocator.dart";
 

thermion_dart/lib/thermion_dart/compatibility/web/interop/thermion_viewer_wasm.dart

@@ -33,8 +33,11 @@ extension type _EmscriptenModule(JSObject _) implements JSObject {
   external JSAny get HEAP32;
 }
 
+typedef ThermionViewerImpl = ThermionViewerWasm;
+
+
 ///
-/// An [ThermionViewer] implementation that forwards calls to
+/// A [ThermionViewer] implementation that forwards calls to
 /// the (Emscripten-generated) ThermionDart JS module.
 ///
 class ThermionViewerWasm implements ThermionViewer {
@@ -43,8 +46,8 @@ class ThermionViewerWasm implements ThermionViewer {
   bool _initialized = false;
   bool _rendering = false;
 
-  ThermionViewerWasm({String moduleName = "thermion_dart"}) {
-    _module = window.getProperty<_EmscriptenModule>(moduleName.toJS);
+  ThermionViewerWasm({JSObject? module, String moduleName = "thermion_dart"}) {
+    _module = module as _EmscriptenModule? ?? window.getProperty<_EmscriptenModule>(moduleName.toJS);
   }
 
   JSNumber? _viewer;
@@ -1687,7 +1690,7 @@ class ThermionViewerWasm implements ThermionViewer {
   Future setMorphTargetWeights(
       ThermionEntity entity, List<double> weights) async {
     final numWeights = weights.length;
-    final ptr = _module._malloc(numWeights * 4) as JSNumber;
+    final ptr = _module._malloc(numWeights * 4);
     for (int i = 0; i < numWeights; i++) {
       _module.setValue(
           (ptr.toDartInt + (i * 4)).toJS, weights[i].toJS, "float");