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/call.rs

@@ -9,7 +9,7 @@ fn bench_call(c: &mut Criterion) {
     let js = "function fib(n) { if (n < 2) return n; return fib(n-1) + fib(n-2); } fib(15);";
     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("call", |b| {
         b.iter_batched(