Return per-statement results from batch()

[penberg]

Make the JavaScript batch() return an array of ResultSet, one per input
statement, matching the libSQL client contract. Each ResultSet carries
that statement's columns, columnTypes, rows (with both positional and
named access), rowsAffected, and a BigInt lastInsertRowid.

Previously the native Database.batch() returned an aggregate
{ rowsAffected, lastInsertRowid } and discarded rows, while the
libSQL-compat client.batch() ignored statement args and collapsed all
statements into a single result. This reworks every layer:

• Native Database.batch() reads each statement's columns, collects rows
  in raw mode, and builds one ResultSet per statement.
• session.batch() requests rows (want_rows) and returns one raw result
  per statement, preserving the atomic Hrana condition chain.
• Connection.batch() shapes each result into a libSQL ResultSet.
• compat client.batch() binds args and runs the batch atomically.

Add conformance tests for both the native Database.batch() and the
libSQL-compat client.batch().