fix: keep the decimal point when rounding gains a digit

[SashaMIT]

Summary

fmt.Sprintf("%.2f", d) for 9.996208266660 printed 1.000 instead of 10.00. The same path printed 0.099999999999999999999999999999998 with %.6f as 0.010000 instead of 0.100000.

When rounding carries from 999 to 1000, roundString keeps the extra digit and leaves the caller to place the point. The formatter treated that digit as a discarded place, so the point moved one position left and the displayed value was ten times too small. Values below 1 then dropped that carry digit as well.

9.94 with %.1f is still 9.9. 1.25 with %.1f is still 1.2. 2.5 with %.0f is still 2.

Test plan

• TestDecimal_Format fails on the old formatter (1.000 vs 10.00) and passes with this change
• go test -count=1 -run 'TestDecimal_Format|TestRoundString|TestBig_Format|TestDecomposer'

Fixes #178
Fixes #198