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

crates/oxide_emit/src/expr/this.rs

@@ -0,0 +1,14 @@
+use crate::{CompileCtx, Emitter};
+use oxide_ir::inst::Inst;
+use oxide_ir::operand::Operand;
+use oxide_bytecode::opcode::OpCode;
+
+impl Emitter {
+    pub(crate) fn emit_this_expression(&self, ctx: &mut CompileCtx) -> Result<u8, String> {
+        let r = ctx.alloc_reg();
+        let src = ctx.static_block_this_reg.unwrap_or(254);
+        let src_op = if src == 254 { Operand::This } else { Operand::Reg(src as u32) };
+        ctx.inst(Inst::new(OpCode::LOAD_VAR, Operand::Reg(r as u32), src_op, Operand::None));
+        Ok(r)
+    }
+}