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

crates/oxide_emit/src/stmt/basic/mod.rs

@@ -9,7 +9,7 @@ mod labeled;
 mod return_stmt;
 
 impl Emitter {
-    pub(crate) fn emit_basic_domain(&self, stmt: &Statement, ctx: &mut CompileCtx) -> Result<Option<u8>, String> {
+    pub(crate) fn emit_basic_domain(&self, stmt: &Statement, ctx: &mut CompileCtx) -> Result<Option<u32>, String> {
         match stmt {
             Statement::ExpressionStatement(_) => self.emit_basic_expression(stmt, ctx),
             Statement::ReturnStatement(_) => self.emit_basic_return(stmt, ctx),