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

crates/oxide_emit/src/stmt/switch.rs

@@ -0,0 +1,51 @@
+use crate::{CompileCtx, Emitter};
+use oxide_ir::inst::Inst;
+use oxide_ir::operand::Operand;
+use oxide_bytecode::opcode::OpCode;
+use oxide_parser::Statement;
+
+impl Emitter {
+    fn emit_switch_statement(&self, stmt: &Statement, ctx: &mut CompileCtx) -> Result<Option<u8>, String> {
+        let Statement::SwitchStatement(sw) = stmt else {
+            return Ok(None);
+        };
+        let end_label = ctx.next_label_id();
+        ctx.push_switch(end_label);
+        let disc_reg = self.emit_expression(&sw.discriminant, ctx)?;
+        let compare_reg_checkpoint = ctx.reg_checkpoint();
+        let cases = &sw.cases;
+        let mut case_labels = Vec::with_capacity(cases.len());
+        for case in cases.iter() {
+            let case_label = ctx.next_label_id();
+            case_labels.push(case_label);
+            if let Some(test) = &case.test {
+                let test_reg = self.emit_expression(test, ctx)?;
+                let eq_reg = ctx.alloc_reg();
+                ctx.inst(Inst::new(OpCode::EQ, Operand::Reg(eq_reg as u32), Operand::Reg(disc_reg as u32), Operand::Reg(test_reg as u32)));
+                ctx.inst(Inst::jmp_if_true(eq_reg, case_label));
+                ctx.restore_reg_checkpoint(compare_reg_checkpoint);
+            }
+        }
+        let has_default = cases.iter().any(|c| c.test.is_none());
+        if !has_default {
+            ctx.inst(Inst::jmp(end_label));
+        }
+        for (case_idx, case) in cases.iter().enumerate() {
+            let case_label = case_labels[case_idx];
+            ctx.labels.set_label_pos(case_label, ctx.insts.len());
+            for s in &case.consequent {
+                self.emit_statement(s, ctx)?;
+            }
+        }
+        ctx.labels.set_label_pos(end_label, ctx.insts.len());
+        ctx.pop_switch();
+        Ok(None)
+    }
+
+    pub(crate) fn emit_switch_domain(&self, stmt: &Statement, ctx: &mut CompileCtx) -> Result<Option<u8>, String> {
+        match stmt {
+            Statement::SwitchStatement(_) => self.emit_switch_statement(stmt, ctx),
+            _ => Ok(None),
+        }
+    }
+}