Fraction.add and subtract return the reduced form when an operand is zero

[lenamonj]

Follow-up to #1784. The zero shortcuts in addSub returned the other operand as constructed, so ZERO.add(2/4) was 2/4 against the Javadoc's reduced-form promise, and ZERO.subtract(Integer.MIN_VALUE/2) threw although the reduced result 1073741824/1 fits, both noted in the #1784 review. The shortcuts now reduce before returning or negating. The new test covers both operations with zero on either side and the Integer.MIN_VALUE boundary; it fails on master and passes with the change. Default mvn goal green with -Ddoclint=all.

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[garydgregory]

@lenamonj Looks good, thank you, merged 🚀