PostgreSQL rejects \\x00 (null byte) in UTF-8 text strings with:
  SQLSTATE[22021]: invalid byte sequence for encoding \"UTF8\": 0x00

Replace the null byte column separator with \\x1f (Unit Separator, ASCII 31)
which is valid UTF-8 and equally unlikely to appear in real data.

Affected drivers:
- PostgreSQL: E'\\x00' → E'\\x1f' in md5() hash expression
- SQLite: X'00' → X'1f' in hex()/sha3() hash expressions

Also re-record the 7 SQLite expected-output baselines that were previously
deleted (fixture data was updated to include real data differences)."

LocalTableData::getDiff (4 returns → 3):
  Extract MySQL CHECKSUM + diff logic into getDiffMySQL(), leaving
  getDiff() with only 3 exit points (sqlite, pgsql, mysql).

StreamingMergeDiff::getDiff (cognitive complexity 49 → low):
  Extract collectInserts(), collectDeletes(), collectUpdates(), and
  buildSingleUpdate() so that getDiff() is a thin orchestrator with no
  nested loops. Each helper has a single clear responsibility.

StreamingMergeDiff::buildHashExpression (4 returns → 2):
  Replace switch/case with a match expression that returns a single
  computed value, reducing return statements from 4 to 2.

StreamingMergeDiff: dedicated exception (RuntimeException → DataException):
  Replace generic \RuntimeException with DBDiff\Exceptions\DataException
  in the unsupported-driver default branch."

CONCAT() returns NULL if any argument is NULL. Without COALESCE, any
row whose hash expression contained a NULL column produced a NULL hash.
In Phase 1 of the streaming merge, PHP compares hashes with !==; since
null !== null evaluates to false, a changed row where both source and
target shared the same NULL column(s) but differed in other columns
would silently pass through as identical — the update would be missed.

Also add CHAR(31) as the column separator (consistent with the pgsql
and sqlite drivers, which already use \x1f) to prevent cross-column
hash collisions where different value distributions produce the same
concatenated string.

Fix: wrap each column in COALESCE(CAST(col AS CHAR CHARACTER SET utf8), '')
and separate columns with CHAR(31) in the CONCAT call.

Updated testBuildHashExpressionMySQL to assert COALESCE and CHAR(31)
are both present in the generated expression."