refactor: 寄存器号 u8→u32 全链放宽，vreg 化去 checkpoint/reg_overflow

crates/oxide_emit/src/class/field.rs

@@ -10,19 +10,22 @@ impl Emitter {
     /// this.y = 1 } static z = this.y + 1` must run in that order). Emitting
     /// all fields before all blocks would make `z` read `y` before it is set.
     pub(crate) fn emit_class_static_elements(
-        &self, elements: &[ClassElement], ctor_reg: u8, ctx: &mut CompileCtx,
+        &self, elements: &[ClassElement], ctor_reg: u32, ctx: &mut CompileCtx,
     ) -> Result<(), String> {
         let saved_static_this = ctx.static_block_this_reg;
-        ctx.static_block_this_reg = Some(ctor_reg);
+        // static_block_this_reg 保留 u8（this 槽语义，254 特判），ctor_reg 超 u8 界时报错而非静默截断。
+        ctx.static_block_this_reg = Some(
+            u8::try_from(ctor_reg).map_err(|_| format!("class constructor register {ctor_reg} exceeds u8 limit"))?,
+        );
         for element in elements {
             match element {
                 ClassElement::PropertyDefinition(prop) => {
                     let prop = prop.as_ref();
                     if prop.r#static {
                         if let PropertyKey::PrivateIdentifier(private) = &prop.key {
-                            self.emit_private_field_init(Operand::Reg(ctor_reg as u32), private.name.as_str(), prop.value.as_ref(), ctx)?;
+                            self.emit_private_field_init(Operand::Reg(ctor_reg), private.name.as_str(), prop.value.as_ref(), ctx)?;
                         } else {
-                            self.emit_public_field_init(Operand::Reg(ctor_reg as u32), &prop.key, prop.computed, prop.value.as_ref(), ctx)?;
+                            self.emit_public_field_init(Operand::Reg(ctor_reg), &prop.key, prop.computed, prop.value.as_ref(), ctx)?;
                         }
                     }
                 }