feat: 数组+索引 [T;N], arr[i] (P1 #6)

- lexer: TOK_LBRACKET, TOK_RBRACKET
- type: TYPE_ARRAY + TypeInfo扩展(element_type/array_size)
- ast: AST_INDEX_EXPR, AST_ARRAY_ASSIGN_STMT
- parser: parse_type_expr()支持[T;N], Pratt加[索引], 数组元素赋值
- sema: 数组类型检查, 索引必须i64, 元素赋值类型匹配
- codegen: type_info_to_llvm(TYPE_ARRAY), GEP+load/store
- 新增集成测试: 18_array.l

测试: 136 通过 (41+15+59+21)

src/lexer/lexer.c

@@ -137,6 +137,8 @@ Token* lex(Arena* a, const char* source, const char* filename,
         else if (c == '|' && peek_next(&l) == '|') { tokens[idx++] = make_token(&l, TOK_PIPE_PIPE, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '.' && peek_next(&l) == '.') { tokens[idx++] = make_token(&l, TOK_DOT_DOT, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '.') { tokens[idx++] = make_token(&l, TOK_DOT, l.pos, 1); advance(&l); }
+        else if (c == '[') { tokens[idx++] = make_token(&l, TOK_LBRACKET, l.pos, 1); advance(&l); }
+        else if (c == ']') { tokens[idx++] = make_token(&l, TOK_RBRACKET, l.pos, 1); advance(&l); }
         else if (c == '(') { tokens[idx++] = make_token(&l, TOK_LPAREN, l.pos, 1); advance(&l); }
         else if (c == ')') { tokens[idx++] = make_token(&l, TOK_RPAREN, l.pos, 1); advance(&l); }
         else if (c == '{') { tokens[idx++] = make_token(&l, TOK_LBRACE, l.pos, 1); advance(&l); }