feat: 实现默认参数（形参 initializer 与解构默认值）

crates/oxide_emit/src/stmt/declaration/function_decl.rs

@@ -22,12 +22,16 @@ impl Emitter {
         for (idx, param) in fd.params.items.iter().enumerate() {
             match &param.pattern {
                 oxide_parser::BindingPattern::BindingIdentifier(bi) => {
-                    param_names.push(ParamSpec::Identifier(bi.name.to_string()))
+                    param_names.push(ParamSpec::Identifier {
+                        name: bi.name.to_string(),
+                        initializer: param.initializer.as_deref(),
+                    })
                 }
                 pattern => param_names.push(ParamSpec::Pattern {
-                    synthetic_name: format!("@@param_{idx}"),
-                    pattern,
-                }),
+                        synthetic_name: format!("@@param_{idx}"),
+                        pattern,
+                        initializer: param.initializer.as_deref(),
+                    }),
             }
         }
         let body_stmts: &[Statement] = if let Some(body) = &fd.body { &body.statements } else { &[] };
@@ -50,3 +54,4 @@ impl Emitter {
         Ok(None)
     }
 }
+