db.execute() write statements now commit automatically

Raw write statements previously opened an implicit transaction that
stayed open until something committed it - the write was visible on
the same connection, making it look saved, but was silently rolled
back when the connection closed. execute() now commits any implicit
transaction it opens, so raw writes behave like every other write
in the library: committed as soon as they run, unless an explicit
transaction (db.begin() or db.atomic()) is open, in which case they
join it.

Row-returning writes such as INSERT ... RETURNING via db.query()
commit once their rows have been iterated. atomic(), commit() and
rollback() now issue literal COMMIT/ROLLBACK statements, which have
identical semantics in every sqlite3 connection mode. The CLI bulk
command uses db.atomic() instead of 'with db.conn:'.

This simplifies the transaction contract to: everything commits
immediately unless you explicitly opened a transaction. Docs
updated throughout; changelog and upgrading guide cover the
breaking change for code that relied on rolling back uncommitted
execute() writes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UnLnhsH25Nnv7LHhekUfPd

docs/changelog.rst

@@ -11,6 +11,7 @@ Unreleased
 
 Breaking changes:
 
+- Write statements executed with ``db.execute()`` are now committed automatically, unless a transaction is already open in which case they join it. Previously they opened an implicit transaction that stayed open until something committed it - writes appeared to work when read on the same connection but were silently rolled back when the connection closed. Code that relied on rolling back uncommitted ``db.execute()`` writes should use the new ``db.begin()`` method to open an explicit transaction first. The transaction model is documented in full at :ref:`python_api_transactions`.
 - ``db.query()`` now executes its SQL as soon as it is called, rather than waiting until the returned generator is first iterated. Rows are still fetched lazily during iteration. SQL errors are now raised at the call site, statements such as ``INSERT ... RETURNING`` take effect without needing to iterate over their results, and passing a statement that returns no rows - previously a silent no-op - now raises a ``ValueError`` recommending ``db.execute()`` instead.
 - Python API validation errors now raise ``ValueError`` instead of ``AssertionError``. Previously invalid arguments - such as ``create_table()`` with no columns, ``transform()`` on a table that does not exist, or passing both ``ignore=True`` and ``replace=True`` - were rejected using bare ``assert`` statements, which are silently skipped when Python runs with the ``-O`` flag. Code that caught ``AssertionError`` for these cases should catch ``ValueError`` instead.
 - ``table.upsert()`` and ``table.upsert_all()`` now raise ``PrimaryKeyRequired`` if a record is missing a value for any primary key column, or has a value of ``None`` for one. Previously such records - which can never match an existing row - were quietly inserted as brand new rows, or triggered a confusing ``KeyError`` after the insert had already taken place.