Validate AtomicFixnum update results before mutation - #1114

The invalid block result replaces the stored integer in the mutex implementation; the C implementation likewise passed the arbitrary Ruby VALUE through compare-and-set without enforcing the numeric invariant. Later numeric operations then operate on a non-integer value. I added a shared regression example that returns a String, verifies the update raises, and verifies the previous integer remains intact. It runs against both MutexAtomicFixnum and CAtomicFixnum; after compiling the extension, the focused suite passes with 59 examples and 0 failures.