Fix RowBinary tuple encoding re-normalizing nested types

[SAY-5]

Fixes #367.

encode({:tuple, types}, values) called the public encode_row/2, which runs encoding_types/1 again. The tuple's field types were already normalized by encoding_type({:tuple, ts}), so encoding a tuple with a normalized nested type like DateTime64 or Time64 re-applied time_unit/1 to an already-computed value (e.g. 1000) and raised FunctionClauseError.

Switched the tuple clause to the private _encode_row/2, which assumes normalized types and skips the second pass. Added a round-trip regression test for Tuple(DateTime64(3, 'UTC')) and Tuple(Time64(6)).