The first-keyword check (`WITH` → SELECT path) was wrong for CTEs that
precede mutations (e.g. `WITH stale AS (...) DELETE FROM t WHERE ...`).
These would be routed through `db.prepare().all()` instead of executing
the mutation.
Use better-sqlite3's `stmt.reader` property, which asks SQLite's own
parser whether the statement returns data. Single mutations go through
`stmt.run()`; compound statements (which `prepare()` rejects) fall back
to `db.exec()`.
Add a regression test for WITH...DELETE.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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