refactor: 拆分 oxide_emit 为独立 emit pass crate（依赖倒置）

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

@@ -0,0 +1,17 @@
+use crate::{CompileCtx, Emitter};
+use oxide_parser::Statement;
+
+mod class_decl;
+mod function_decl;
+mod var;
+
+impl Emitter {
+    pub(crate) fn emit_declaration_domain(&self, stmt: &Statement, ctx: &mut CompileCtx) -> Result<Option<u8>, String> {
+        match stmt {
+            Statement::VariableDeclaration(_) => self.emit_variable_declaration_statement(stmt, ctx),
+            Statement::FunctionDeclaration(_) => self.emit_function_declaration_statement(stmt, ctx),
+            Statement::ClassDeclaration(_) => self.emit_class_declaration_statement(stmt, ctx),
+            _ => Ok(None),
+        }
+    }
+}