perf: run 签名改 &Arc<CompiledModule>，顶层模块跨 run 共享——collect_flat_modules 顶层改 Arc::clone 消除逐 run 常量池深拷贝，全调用点 Arc 包装迁移

crates/oxide_vm/benches/coercion.rs

@@ -9,7 +9,7 @@ fn bench_coercion(c: &mut Criterion) {
     let js = "var x = \"42\"; var y = \"3.14\"; var r = 0; for (var i = 0; i < 5000; i++) { r += (+x) + (+y); } r";
     let alloc = oxide_parser::Allocator::default();
     let program = oxide_parser::parse(&alloc, js).expect("parse");
-    let module = Compiler::new().compile(&program).expect("compile");
+    let module = Arc::new(Compiler::new().compile(&program).expect("compile"));
 
     c.bench_function("coercion", |b| {
         b.iter_batched(