feat: atomic transactions for Cloudflare Durable Objects SQLite + tx.sql (#140)

[pdlug]

Closes #140.

What ships

Cloudflare Durable Objects SQLite is now fully transactional. A store backed by drizzle(ctx.storage) is auto-detected as a new SQLite transactionMode: "do-sqlite" and reports capabilities.transactions: true (previously false). store.transaction() and store.withTransaction() roll back and commit atomically by delegating to the async ctx.storage.transaction(async …) runner (Drizzle's db.$client.transaction), which rolls back across await. Drizzle's own db.transaction() on DO is ctx.storage.transactionSync and cannot span an await, so it is deliberately not used.

This also fixes a latent detection bug: drizzle's Durable Objects session class is SQLiteDOSession, not the previously-checked SQLiteDurableObjectSession, so a real drizzle(ctx.storage) store was silently misclassified.

TransactionContext.sql — graph-owned cross-store writes (all transactional backends). store.transaction(async (tx) => …) now exposes tx.sql, the raw Drizzle handle bound to the same transaction, so the caller's own relational tables join the same atomic boundary — the graph-owned counterpart of store.withTransaction. On Postgres/libsql this is a correctness requirement (the outer db is a different connection and would escape the transaction); undefined only on non-transactional backends.

await store.transaction(async (tx) => {
  await tx.nodes.Document.update(documentId, props);
  await tx.sql.insert(documentVersions).values(versionRow);
});

Invariants preserved

Schema bootstrap/DDL and the durable fulltext materialization marker run outside any storage transaction; only the data-only schema-version commit uses the storage runner — "no DDL in the business transaction" holds.

Testing

• New pnpm test:do lane (@cloudflare/vitest-pool-workers) driving a real Durable Object in workerd: cross-store commit/rollback, product-key-conflict rollback, same-document contention.
• Graph-owned tx.sql commit/rollback coverage on better-sqlite3, libsql, Postgres, and do-sqlite.
• Full SQLite suite (3105) + do-sqlite (8) + Postgres cross-store (8) green; typecheck/lint/format clean.

Out of scope

Cloudflare D1 stays transactionMode: "none": D1Database.batch(...) is transactional but batch-only, not an interactive runner. A batch-mode D1 path is tracked separately (per the issue's own scoping).