fix: str+str 运行时拼接 — malloc + strlen + memcpy

- codegen: 声明 CRT 的 malloc/strlen/memcpy
- str+str 拼接: strlen(l)+strlen(r)+1 → malloc → memcpy×2 → 返回指针
- 新增集成测试 08_str_concat.l ("Hello, " + "World!" → "Hello, World!")
- 修复自报告 §5-6 字符串拼接不工作的 bug

src/codegen/codegen.c

@@ -31,6 +31,10 @@ typedef struct {
     // printf 运行时支持（内置 print 函数委托给 printf）
     LLVMValueRef      printf_fn;
     LLVMTypeRef       printf_ty;
+    // 字符串拼接运行时支持
+    LLVMValueRef      malloc_fn;
+    LLVMValueRef      strlen_fn;
+    LLVMValueRef      memcpy_fn;
 } CgCtx;
 
 // === 类型映射（需要 Context）===
@@ -124,8 +128,39 @@ static LLVMValueRef codegen_expr(CgCtx* ctx, AstNode* node) {
         LLVMValueRef r = codegen_expr(ctx, node->as.binary.right);
         if (!l || !r) return NULL;
 
-        // 字符串拼接：暂不支持运行时拼接，直接返回左操作数
-        if (node->type.kind == TYPE_STR) return l;
+        // 字符串拼接：alloc 栈缓冲区，strcpy + strcat
+        if (node->type.kind == TYPE_STR) {
+            // strlen(left)
+            LLVMValueRef len_l = LLVMBuildCall2(ctx->builder,
+                LLVMGlobalGetValueType(ctx->strlen_fn), ctx->strlen_fn,
+                (LLVMValueRef[]){l}, 1, "strlen_l");
+            // strlen(right)
+            LLVMValueRef len_r = LLVMBuildCall2(ctx->builder,
+                LLVMGlobalGetValueType(ctx->strlen_fn), ctx->strlen_fn,
+                (LLVMValueRef[]){r}, 1, "strlen_r");
+            // total = len_l + len_r + 1
+            LLVMValueRef total = LLVMBuildAdd(ctx->builder, len_l, len_r, "total");
+            total = LLVMBuildAdd(ctx->builder, total,
+                LLVMConstInt(LLVMInt64TypeInContext(ctx->context), 1, false), "total_1");
+            // char* buf = malloc(total)
+            LLVMValueRef buf = LLVMBuildCall2(ctx->builder,
+                LLVMGlobalGetValueType(ctx->malloc_fn), ctx->malloc_fn,
+                (LLVMValueRef[]){total}, 1, "str_buf");
+            // memcpy(buf, left, len_l)
+            LLVMBuildCall2(ctx->builder,
+                LLVMGlobalGetValueType(ctx->memcpy_fn), ctx->memcpy_fn,
+                (LLVMValueRef[]){buf, l, len_l}, 3, "");
+            // memcpy(buf + len_l, right, len_r + 1)  -- includes null terminator
+            LLVMValueRef offset_ptr = LLVMBuildGEP2(ctx->builder,
+                LLVMInt8TypeInContext(ctx->context), buf,
+                (LLVMValueRef[]){len_l}, 1, "offset");
+            LLVMValueRef len_r1 = LLVMBuildAdd(ctx->builder, len_r,
+                LLVMConstInt(LLVMInt64TypeInContext(ctx->context), 1, false), "len_r1");
+            LLVMBuildCall2(ctx->builder,
+                LLVMGlobalGetValueType(ctx->memcpy_fn), ctx->memcpy_fn,
+                (LLVMValueRef[]){offset_ptr, r, len_r1}, 3, "");
+            return buf;
+        }
 
         bool is_float = (node->type.kind == TYPE_F64);
 
@@ -352,6 +387,29 @@ LLVMModuleRef codegen_module(AstNode* ast, Arena* codegen_arena,
         LLVMInt32TypeInContext(ctx.context), printf_param_types, 1, true);
     ctx.printf_fn = LLVMAddFunction(ctx.module, "printf", ctx.printf_ty);
 
+    // 声明 malloc: void* malloc(size_t)
+    LLVMTypeRef malloc_args[] = { LLVMInt64TypeInContext(ctx.context) };
+    LLVMTypeRef malloc_ty = LLVMFunctionType(
+        LLVMPointerType(LLVMInt8TypeInContext(ctx.context), 0), malloc_args, 1, false);
+    ctx.malloc_fn = LLVMAddFunction(ctx.module, "malloc", malloc_ty);
+
+    // 声明 strlen: size_t strlen(const char*)
+    LLVMTypeRef strlen_args[] = { LLVMPointerType(LLVMInt8TypeInContext(ctx.context), 0) };
+    LLVMTypeRef strlen_ty = LLVMFunctionType(
+        LLVMInt64TypeInContext(ctx.context), strlen_args, 1, false);
+    ctx.strlen_fn = LLVMAddFunction(ctx.module, "strlen", strlen_ty);
+
+    // 声明 memcpy: void* memcpy(void*, const void*, size_t)
+    LLVMTypeRef memcpy_args[] = {
+        LLVMPointerType(LLVMInt8TypeInContext(ctx.context), 0),
+        LLVMPointerType(LLVMInt8TypeInContext(ctx.context), 0),
+        LLVMInt64TypeInContext(ctx.context),
+    };
+    LLVMTypeRef memcpy_ty = LLVMFunctionType(
+        LLVMPointerType(LLVMInt8TypeInContext(ctx.context), 0),
+        memcpy_args, 3, false);
+    ctx.memcpy_fn = LLVMAddFunction(ctx.module, "memcpy", memcpy_ty);
+
     // 第一遍：声明所有 L 函数
     for (size_t i = 0; i < ast->as.program.fn_count; i++) {
         AstNode* fn = ast->as.program.functions[i];