fix(sqlite): correct sub-second decoding of pre-epoch REAL datetimes (#4340)

Datetimes stored as a REAL Julian day number (e.g. via SQLite's
julianday()) were decoded by splitting the UNIX timestamp with
trunc()/fract().abs(). timestamp_opt() always adds the nanoseconds
forward in time, so for values before 1970 this pushed the result up to
~2 seconds off and could move it onto the wrong side of the epoch
(e.g. 1969-12-31 23:59:59.5 decoded as 1970-01-01 00:00:00.5).

Round seconds toward negative infinity with floor() and take the
sub-second remainder relative to that, so nanos is always a valid
forward offset. Post-epoch values are unaffected. Adds unit tests for
the pre- and post-epoch paths.