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,

crates/oxide_runtime_api/src/lib.rs

@@ -193,6 +193,54 @@ pub fn to_int32(val: JsValue) -> i32 {
     }
 }
 
+/// Append the string representation of `val` to `buf` without allocating
+/// intermediate temporary Strings. Used in hot-path string concatenation.
+pub fn push_to_string(val: JsValue, buf: &mut String) {
+    if val.is_int() {
+        use std::fmt::Write;
+        let _ = write!(buf, "{}", val.as_int());
+        return;
+    }
+    if val.is_double() {
+        let d = val.as_double();
+        if d.is_nan() {
+            buf.push_str("NaN");
+            return;
+        }
+        if d.is_infinite() {
+            buf.push_str(if d.is_sign_positive() { "Infinity" } else { "-Infinity" });
+            return;
+        }
+        if d.is_finite() && d.fract() == 0.0 {
+            use std::fmt::Write;
+            let _ = write!(buf, "{}", d as i64);
+            return;
+        }
+        let mut ryubuf = ryu::Buffer::new();
+        buf.push_str(ryubuf.format(d));
+        return;
+    }
+    if val.is_bool() {
+        buf.push_str(if val.as_bool() { "true" } else { "false" });
+        return;
+    }
+    if val.is_null() {
+        buf.push_str("null");
+        return;
+    }
+    if val.is_undefined() {
+        buf.push_str("undefined");
+        return;
+    }
+    if val.is_string() {
+        unsafe { buf.push_str(string_data(val)) };
+        return;
+    }
+    if val.is_object() {
+        buf.push_str("[object]");
+    }
+}
+
 pub fn to_string(val: JsValue) -> String {
     if val.is_int() {
         return val.as_int().to_string();

crates/oxide_vm/src/dispatch/arithmetic.rs

@@ -10,10 +10,12 @@ impl Vm {
         let lhs = self.coerce_primitive_bounded(self.regs[a], false)?;
         let rhs = self.coerce_primitive_bounded(self.regs[b], false)?;
         if lhs.is_string() || rhs.is_string() {
-            let ls = coercion::to_string(lhs);
-            let rs = coercion::to_string(rhs);
-            let concat = format!("{ls}{rs}");
-            self.regs[rd] = self.new_string(&concat);
+            let mut buf = std::mem::take(&mut self.string_buf);
+            buf.clear();
+            coercion::push_to_string(lhs, &mut buf);
+            coercion::push_to_string(rhs, &mut buf);
+            self.regs[rd] = self.new_string(&buf);
+            self.string_buf = buf;
         } else {
             let ln = coercion::to_number(lhs);
             let rn = coercion::to_number(rhs);
@@ -43,10 +45,12 @@ impl Vm {
         let lhs = self.coerce_primitive_bounded(self.regs[rd], false)?;
         let rhs = self.coerce_primitive_bounded(self.regs[a], false)?;
         if lhs.is_string() || rhs.is_string() {
-            let ls = coercion::to_string(lhs);
-            let rs = coercion::to_string(rhs);
-            let concat = format!("{ls}{rs}");
-            self.regs[rd] = self.new_string(&concat);
+            let mut buf = std::mem::take(&mut self.string_buf);
+            buf.clear();
+            coercion::push_to_string(lhs, &mut buf);
+            coercion::push_to_string(rhs, &mut buf);
+            self.regs[rd] = self.new_string(&buf);
+            self.string_buf = buf;
         } else {
             let ln = coercion::to_number(lhs);
             let rn = coercion::to_number(rhs);

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 {

crates/oxide_vm/src/vm_support.rs

@@ -69,6 +69,7 @@ impl Vm {
                 ic_misses: std::cell::Cell::new(0),
                 instruction_count: 0,
             },
+            string_buf: String::new(),
         };
         vm_info!("Vm created");
         vm
@@ -127,6 +128,7 @@ impl Vm {
                 ic_misses: std::cell::Cell::new(0),
                 instruction_count: 0,
             },
+            string_buf: String::new(),
         };
         vm_info!("Vm created (pool)");
         vm
@@ -220,9 +222,15 @@ impl Vm {
     }
 
     pub fn new_string(&mut self, s: &str) -> JsValue {
-        let ptr = Box::into_raw(Box::new(JsString::new(s.to_string())));
+        self.new_string_owned(s.to_string())
+    }
+
+    /// Same as `new_string` but takes `String` by move, avoiding a clone.
+    pub fn new_string_owned(&mut self, s: String) -> JsValue {
+        let len = s.len();
+        let ptr = Box::into_raw(Box::new(JsString::new(s)));
         self.gc_state.session_string_ptrs.push(ptr);
-        self.gc_state.session_bytes_allocated += std::mem::size_of::<JsString>() + s.len();
+        self.gc_state.session_bytes_allocated += std::mem::size_of::<JsString>() + len;
         JsValue::string(ptr)
     }