feat: let mut + 赋值语句 — while 循环可修改变量

- lexer: 新增 TOK_MUT 关键字
- ast: AST_ASSIGN_STMT 节点 + let_stmt.is_mut 标志
- parser: ‘let mut’ 前缀识别 + ‘ident = expr;’ 赋值语句
- sema: Symbol.is_mut 可变性检查（不可变变量赋值报错）
- codegen: AST_ASSIGN_STMT → store 指令
- 新增集成测试 06_mut_while.l（while 循环 + 计数器）

基于 Codex 分析报告 P0 建议。

src/ast/ast.h

@@ -10,6 +10,7 @@ typedef enum {
     AST_PARAMETER,
     AST_BLOCK,
     AST_LET_STMT,
+    AST_ASSIGN_STMT,
     AST_IF_STMT,
     AST_WHILE_STMT,
     AST_RETURN_STMT,
@@ -52,7 +53,9 @@ struct AstNode {
         // AST_BLOCK
         struct { struct AstNode** stmts; size_t stmt_count; } block;
         // AST_LET_STMT
-        struct { const char* name; TypeKind annot_type; bool has_type_annot; struct AstNode* init; } let_stmt;
+        struct { const char* name; TypeKind annot_type; bool has_type_annot; bool is_mut; struct AstNode* init; } let_stmt;
+        // AST_ASSIGN_STMT
+        struct { const char* name; struct AstNode* value; } assign_stmt;
         // AST_IF_STMT
         struct { struct AstNode* cond; struct AstNode* then_block; struct AstNode* else_block; } if_stmt;
         // AST_WHILE_STMT
@@ -80,7 +83,8 @@ AstNode* ast_make_function(void* alloc, const char* name, AstNode** params, size
                            TypeKind ret, AstNode* body, int line, int col);
 AstNode* ast_make_parameter(void* alloc, const char* name, TypeKind type, int line, int col);
 AstNode* ast_make_block(void* alloc, AstNode** stmts, size_t count, int line, int col);
-AstNode* ast_make_let(void* alloc, const char* name, TypeKind annot_type, bool has_type_annot, AstNode* init, int line, int col);
+AstNode* ast_make_let(void* alloc, const char* name, TypeKind annot_type, bool has_type_annot, bool is_mut, AstNode* init, int line, int col);
+AstNode* ast_make_assign(void* alloc, const char* name, AstNode* value, int line, int col);
 AstNode* ast_make_if(void* alloc, AstNode* cond, AstNode* then_b, AstNode* else_b, int line, int col);
 AstNode* ast_make_while(void* alloc, AstNode* cond, AstNode* body, int line, int col);
 AstNode* ast_make_return(void* alloc, AstNode* expr, int line, int col);