Round away from zero in ROUND_UP for values far below the rounding position

[Shaisolaris]

round(n, :up) returns zero for a nonzero value that lies more than one internal digit word (9 decimal digits) below the rounding position:

BigDecimal("1e-7").round(0, :up)    # => 1
BigDecimal("1e-10").round(0, :up)   # => 0, expected 1
BigDecimal("1e-10").round(6, :up)   # => 0.0, expected 0.000001
BigDecimal("1e-10").ceil(6)         # => 0.000001

VpMidRound truncates everything when the rounding position is left of the first word, but keeps going for ROUND_CEILING and ROUND_FLOOR so they can round to the next unit. ROUND_UP needs the same treatment, since any nonzero remainder rounds away from zero. The change adds it to that check; the existing path (exptoadd) then produces the right result.

Testing:

• New test_round_up_value_far_below_rounding_position fails before the change and passes after.
• All files under test/bigdecimal pass with the change (270 tests, 0 failures, 13 omissions) on Ruby 3.3.8, Linux x86_64. test_sig and JRuby were not run.
• A randomized comparison of round(n, mode) against exact Rational rounding for all seven modes (60,000 cases, exponents -60..40, n -45..45) found 4,800 :up mismatches before and none after.