UPDATE metadata semantics incorrect

The Postgres response for an `UPDATE` reports how many rows matched the WHERE clause, regardless of whether the value changed. This differs from the MySQL semantics (reports rows changed), but is equivalent to setting the `CLIENT_FOUND_ROWS` client compatibility flag.

This bug is critical due to its interpretation by ORMs, where a 0 result is used as proof of absence from a table.

```sql
CREATE TABLE bug3 (id integer PRIMARY KEY, v text);
INSERT INTO bug3 VALUES (1, 'same');

UPDATE bug3 SET v = 'same' WHERE id = 1; -- reports 0; PostgreSQL reports 1
UPDATE bug3 SET v = 'different' WHERE id = 1; -- reports 1
UPDATE bug3 SET v = 'x' WHERE id = 999; -- reports 0
```

Start by running the SQL reproduction in the issue and tracing the UPDATE result-count handling in the Go code. Verify the behavior for unchanged values, changed values, and unmatched rows; done means the reported count follows PostgreSQL semantics in all three cases.

Written by the indexing model from the issue text.