planner: TIME column comparisons with temporal values use incompatible coercion semantics

## Bug Report

Comparing a `TIME` column with a `DATETIME` value may return a result inconsistent with MySQL because the date part of the `DATETIME` value affects the comparison.

### 1. Minimal reproduce step (Required)

```sql
CREATE TABLE t (
id INT PRIMARY KEY,
trx_time TIME
);

INSERT INTO t VALUES (1, '19:10:06');

SELECT id
FROM t
WHERE trx_time = CAST('1000-01-01 19:10:06' AS DATETIME);
```

### 2. What did you expect to see? (Required)

The query returns the row, consistent with MySQL:

```text
+----+
| id |
+----+
| 1 |
+----+
```

### 3. What did you see instead (Required)

The query returns an empty result.

### 4. What is your TiDB version? (Required)

```sql
mysql> SELECT tidb_version();
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v9.0.0-beta.2.pre-2156-ga80e3356c4
Edition: Community
Git Commit Hash: a80e3356c4befd6c4f316586b0f8e1bd5d24eb1f
Git Branch: master
UTC Build Time: 2026-08-31 05:56:58
GoVersion: go1.25.12
Race Enabled: false
Check Table Before Drop: false
Store: tikv
Kernel Type: Classic |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.001 sec)
```

The issue names no files or tests. Start by running the provided CREATE TABLE, INSERT, and SELECT reproduction in TiDB, then compare the TIME-versus-DATETIME coercion path with MySQL behavior. Done means the query returns the inserted row, matching the expected result.

Written by the indexing model from the issue text.