docs(native-class): update memory layout documentation and add padding fields to test classes

- Add $padding field to NativeFailedClonePressure class in failed-clone-finalizer.phpt
- Add $padding field to NativeFailedLifecyclePressure class in failed-lifecycle-escape.phpt
- Add $padding field to NativeFinalizerFiller class in finalizer-allocation.phpt
- Add $padding field to NativeFinalizerPressureObject class in finalizer-inheritance-exception.phpt
- Add multiple padding fields ($padding1, $padding2, $padding3) to NativeGcThresholdValue class in gc-threshold.phpt
- Add $padding field to NativeLifecycleFiller class in lifecycle-exceptions.phpt
- Update native heap memory layout documentation with fixed header size information
- Clarify that GC header uses two machine words (16 bytes on 64-bit platforms)
- Explain that object size, alignment and callbacks are stored in static NativeTypeDescriptor
- Remove mention of initial

docs/NATIVE_CLASS_OBJECT.md

@@ -431,21 +431,20 @@ Native Class Object 应使用独立的、非移动、精确 tracing GC。本文
 
 ### 8.1 Native Heap
 
-首版 Wren 派生实现为每个对象分配一块连续内存，并在 struct 前放置隐藏 GC header：
+Wren 派生实现为每个对象分配一块连续内存，并在 struct 前放置隐藏 GC header。Header 固定为两个机器字，在 64 位平台上是 16 字节：
 
 ```cpp
 struct NativeGcHeader {
+    // 低 3 位复用为 marked/finalized/allocated-during-collection 标志。
+    uintptr_t nextAndFlags;
     const NativeTypeDescriptor *type;
-    NativeGcHeader *next;
-    uint32_t flags;
-    uint32_t size;
 };
 
 // 内存布局：[NativeGcHeader][php_app__point]
 auto *point = native_heap.make<php_app__point>();
 ```
 
-GC header 不属于生成的 C++ struct，也不会改变属性偏移。用户可见对象变量仍然只是一个 `native_struct *`。
+GC header 不属于生成的 C++ struct，也不会改变属性偏移。用户可见对象变量仍然只是一个 `native_struct *`。对象尺寸、对齐方式及 trace/finalize/destroy 回调保存在每个类型唯一的静态 `NativeTypeDescriptor` 中，不在每个实例中重复保存。
 
 当前分配器使用独立 non-moving allocation；Arena/chunk/free-list 可以作为后续分配器
 优化，但不得改变对象地址稳定性、header 布局、精确 tracing 或 finalization 语义。

tests/compiler/native-class/failed-clone-finalizer.phpt

@@ -25,6 +25,7 @@ class NativeFailedCloneFinalizer
 class NativeFailedClonePressure
 {
     public int $value;
+    public int $padding;
 }
 
 function forceFailedCloneCollection(): void

tests/compiler/native-class/failed-lifecycle-escape.phpt

@@ -68,6 +68,7 @@ class NativeFailedCloneEscape
 class NativeFailedLifecyclePressure
 {
     public int $value;
+    public int $padding;
 }
 
 function forceNativeCollection(): void

tests/compiler/native-class/finalizer-allocation.phpt

@@ -28,6 +28,7 @@ class NativeFinalizerCreator
 class NativeFinalizerFiller
 {
     public int $value;
+    public int $padding;
 }
 
 function main(): void

tests/compiler/native-class/finalizer-inheritance-exception.phpt

@@ -28,6 +28,7 @@ class NativeThrowingFinalizerChild extends NativeThrowingFinalizerBase
 class NativeFinalizerPressureObject
 {
     public int $value;
+    public int $padding;
 }
 
 function collectThrowingFinalizer(): void

tests/compiler/native-class/gc-threshold.phpt

@@ -6,6 +6,12 @@ Native class: long-running allocation triggers automatic tracing collection
 #[Native]
 class NativeGcThresholdValue
 {
+    // Keep the pressure in the object payload so this test does not depend on
+    // the implementation size of the hidden Native GC header.
+    public int $padding1;
+    public int $padding2;
+    public int $padding3;
+
     public function __destruct()
     {
         global $nativeGcFinalized;

tests/compiler/native-class/lifecycle-exceptions.phpt

@@ -66,6 +66,7 @@ class NativeThrowingFinalizer
 class NativeLifecycleFiller
 {
     public int $value;
+    public int $padding;
 }
 
 function allocateUntilFinalizerRuns(): void