feat(fltk): main2 演示风格切换（Style 按钮循环 base/plastic/gtk+/gleam/oxy，状态栏 f-string 显示当前风格）

ViperFLProject/App/main2.vp

@@ -16,6 +16,7 @@ import fltk
 # 拿不到 main 的局部变量）；空指针用 None（C null），不能用整数 0。
 term_ptr: t.CPtr = None
 status_ptr: t.CPtr = None
+scheme_idx: int = 0
 
 
 def on_btn_click(w: bytes, data: bytes):
@@ -60,6 +61,24 @@ def on_menu_about(w: bytes, data: bytes):
     fltk.Fl_Output_set_value(status_ptr, "Menu: Help/About")
 
 
+def on_style(w: bytes, data: bytes):
+    """风格切换回调：base→plastic→gtk+→gleam→oxy 循环"""
+    global scheme_idx
+    scheme_idx = scheme_idx + 1
+    if scheme_idx == 1:
+        fltk.Fl.set_scheme("plastic")
+    elif scheme_idx == 2:
+        fltk.Fl.set_scheme("gtk+")
+    elif scheme_idx == 3:
+        fltk.Fl.set_scheme("gleam")
+    elif scheme_idx == 4:
+        fltk.Fl.set_scheme("oxy")
+    else:
+        fltk.Fl.set_scheme("base")
+        scheme_idx = 0
+    fltk.Fl_Output_set_value(status_ptr, f"Scheme: {fltk.Fl.scheme_string()}")
+
+
 def on_tick(w: bytes, data: bytes):
     """定时器回调：每秒终端打印一次并续订"""
     fltk.Fl_Terminal_append(term_ptr, "tick\n")
@@ -76,8 +95,9 @@ def main() -> int:
     btn = fltk.Fl_Button(10, 70, 120, 30, "Click Me")
     check = fltk.Fl_Check_Button(150, 70, 100, 30, "Enable")
     slider = fltk.Fl_Slider(280, 70, 200, 30)
-    input_ = fltk.Fl_Input(10, 110, 300, 30, "Type here")
-    go = fltk.Fl_Button(320, 110, 60, 30, "Go")
+    input_ = fltk.Fl_Input(10, 110, 240, 30, "Type here")
+    go = fltk.Fl_Button(260, 110, 60, 30, "Go")
+    style = fltk.Fl_Button(330, 110, 60, 30, "Style")
     spinner = fltk.Fl_Spinner(400, 110, 120, 30)
     browser = fltk.Fl_Browser(10, 150, 280, 140)
     tree = fltk.Fl_Tree(300, 150, 280, 140)
@@ -97,6 +117,7 @@ def main() -> int:
     check.set_callback(on_check, 0)
     slider.set_callback(on_slider, 0)
     go.set_callback(on_go, input_._ptr)
+    style.set_callback(on_style, 0)
 
     # 数值微调框
     spinner.set_minimum(0.0)