docs(design):anony fntype as field mapped named fntype and refer

doc/en/dev/llcppg.md

@@ -94,7 +94,10 @@ func (recv_ *Sqlite3) Exec(sql *c.Char, callback func(c.Pointer, c.Int, **c.Char
 }
 ```
 
-For struct fields that are anonymous function pointers, the field type is replaced with a `c.Pointer` for description.
+LLGo cannot use anonymous function types directly as field types. To preserve type information, llcppg automatically generates named function types and references them in the field and following this naming convention:
+```
+LLGO_<namespaces>_<typename>_<nested_field_typename>_<fieldname>
+```
 
 ```c
 typedef struct Hooks {
@@ -103,9 +106,60 @@ typedef struct Hooks {
 } Hooks;
 ```
 ```go
+// llgo:type C
+type LLGO_Hooks_MallocFn func(c.SizeT) c.Pointer
+// llgo:type C
+type LLGO_Hooks_FreeFn func(c.Pointer)
+
+type Hooks struct {
+	MallocFn LLGO_Hooks_MallocFn
+	FreeFn   LLGO_Hooks_FreeFn
+}
+```
+
+with namespace
+
+```c
+namespace A {
+  struct Hooks {
+      void *(*malloc_fn)(size_t sz);
+      void (*free_fn)(void *ptr);
+  } Hooks;
+}
+```
+```go
+// llgo:type C
+type LLGO_A_Hooks_MallocFn func(c.SizeT) c.Pointer
+// llgo:type C
+type LLGO_A_Hooks_FreeFn func(c.Pointer)
+
 type Hooks struct {
-	MallocFn c.Pointer
-	FreeFn   c.Pointer
+	MallocFn LLGO_A_Hooks_MallocFn
+	FreeFn   LLGO_A_Hooks_FreeFn
+}
+```
+in nested struct
+
+```c
+struct Foo {
+    struct {
+        void *(*malloc_fn)(size_t sz);
+        void (*free_fn)(void *ptr);
+    } Hooks;
+};
+```
+```go
+// llgo:type C
+type LLGO_Foo_Hooks_MallocFn func(c.SizeT) c.Pointer
+
+// llgo:type C
+type LLGO_Foo_Hooks_FreeFn func(c.Pointer)
+
+type Foo struct {
+	Hooks struct {
+		MallocFn LLGO_Foo_Hooks_MallocFn
+		FreeFn   LLGO_Foo_Hooks_FreeFn
+	}
 }
 ```