Window functions with ORDER BY on a non-output column return rows in nondeterministic order

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
CREATE TABLE wm(v INT, g INT);
INSERT INTO wm VALUES (1,1),(2,1),(5,1),(3,2),(3,2);
SELECT v, COUNT(v) OVER (PARTITION BY g ORDER BY v) c FROM wm ORDER BY g, v;
-- row order varies between executions (expected: 1,2,5,3,3)
```

### 2. What did you expect to see? (Required)
When the outer ORDER BY keys exactly match the window's internal sort keys (all ASC) and reference a column absent from the SELECT list, the planner eliminates the outer Sort (EXPLAIN: 1 Sort in the ASC plan vs 2 in the DESC plan), but the parallel Window executor (Shuffle) does not preserve order. Measured: 2 orderings in 12 runs at 5 rows; up to 6 distinct orderings in 6 runs at 500 rows. Affects all window functions. ORDER BY g DESC, v keeps the Sort and is always correct.

(expected: 1,2,5,3,3)

### 3. What did you see instead (Required)
```
mysql> SELECT v, COUNT(v) OVER (PARTITION BY g ORDER BY v) c FROM wm ORDER BY g, v;
+------+------+
| v | c |
+------+------+
| 1 | 1 |
| 2 | 2 |
| 5 | 3 |
| 3 | 2 |
| 3 | 2 |
+------+------+
5 rows in set (0.00 sec)

mysql> SELECT v, COUNT(v) OVER (PARTITION BY g ORDER BY v) c FROM wm ORDER BY g, v;
+------+------+
| v | c |
+------+------+
| 3 | 2 |
| 3 | 2 |
| 1 | 1 |
| 2 | 2 |
| 5 | 3 |
+------+------+
5 rows in set (0.00 sec)
```
### 4. What is your TiDB version? (Required)

```
mysql> SELECT tidb_version()
-> ;
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v9.0.0-beta.2.pre-2174-g65ac2fad58-dirty
Edition: Community
Git Commit Hash: 65ac2fad582510b92d3c4b79999e48217c989737
Git Branch: master
UTC Build Time: 2026-08-30 15:18:00
GoVersion: go1.25.12
Race Enabled: false
Check Table Before Drop: false
Store: unistore
Kernel Type: Classic |
+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

Reproduce the query repeatedly and compare the ASC and DESC EXPLAIN plans, then trace the planner's outer Sort elimination and the parallel Window executor (Shuffle). Done means ORDER BY g, v remains deterministic when g and v are absent from the SELECT list, with coverage for the reported window-function case.

Written by the indexing model from the issue text.