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_kernel/src/kernel.rs

@@ -39,7 +39,7 @@ impl KernelConfig {
             max_dead_strings: Some(10_000),
             max_steps: None,
             max_call_depth: 1024,
-            session_gc_threshold: 8_388_608,
+            session_gc_threshold: 33_554_432,
             max_cached_modules: 512,
             log_levels: [Level::Off; SUBSYSTEM_COUNT],
             warmup_builtin_shapes: true,
@@ -55,7 +55,7 @@ impl KernelConfig {
             max_dead_strings: Some(10_000),
             max_steps: None,
             max_call_depth: 1024,
-            session_gc_threshold: 8_388_608,
+            session_gc_threshold: 33_554_432,
             max_cached_modules: 512,
             log_levels: [Level::Off; SUBSYSTEM_COUNT],
             warmup_builtin_shapes: true,
@@ -71,7 +71,7 @@ impl KernelConfig {
             max_dead_strings: Some(5_000),
             max_steps: None,
             max_call_depth: 1024,
-            session_gc_threshold: 8_388_608,
+            session_gc_threshold: 33_554_432,
             max_cached_modules: 512,
             log_levels: [Level::Off; SUBSYSTEM_COUNT],
             warmup_builtin_shapes: true,