perf(vm): 字符串拼接用 push_to_string 消去中间分配, GC阈值 8M→32M

- oxide_runtime_api 新增 push_to_string: 直接将 JsValue 写入 String buffer
- dispatch_add / dispatch_compound_add 复用 Vm::string_buf, 每次concat从4次分配降到1次
- vm_support 新增 new_string_owned 接受 String 转移所有权
- kernel session_gc_threshold 8MB→32MB 减少频繁 GC
- build.sh: make clean 清跨平台 .obj 残留

(cherry picked from commit ad7b044a5a24b23819d783c5419d3e33f936dfc5)

crates/oxide_vm/src/vm.rs

@@ -189,6 +189,8 @@ pub struct Vm {
     pub(crate) iters: IterState,
     /// Grouped inline-cache and instruction counters.
     pub(crate) profiling: ProfilingState,
+    /// Reusable string buffer for concatenation to avoid allocation per `+` op.
+    pub(crate) string_buf: String,
 }
 
 impl Vm {