Portable upsert on SQLite and MySQL, plus an excluded helper #81

Signal

onConflictDoUpdate / onConflictDoNothing exist for Postgres. We want them on all dialects:

• SQLite / D1: INSERT … ON CONFLICT(cols) DO UPDATE SET … = excluded.col.
• MySQL: INSERT … ON DUPLICATE KEY UPDATE col = VALUES(col) (or the row alias on MySQL 8.0.19+). Refuse an explicit conflict target, because MySQL uses all unique keys.
• setExcluded(cols): "set all from excluded" (gap review P1).

Why (MyRemotely)

• 70 upsert and 10 skipDuplicates call sites. Sync from Rentvine, TTLock and Twilio is upsert-heavy.

Part of #52: the CAFE-stack roadmap signal from MyRemotely. This shows the direction we want, and it is not a demand. We can send a PR if you accept the idea.