Mutation coverage: IMPL-log10Reduce

[thedavidmeister]

Adds exact-oracle tests for the first third of LibDecimalFloatImplementation.log10Reduce (range check plus steps 1-4).

testLog10ReduceOneStepExamples holds fixed examples that each take one step i in [1, 4]. Each one sits where a reciprocal off by one, or a dropped borrow in the high-word extraction, moves the floor. The expected quotients are floor(x K / 2^256) from bc. K = 2^256 / 10^(2^-i) is rounded to nearest and derived independently with bc -l at scale 200. testLog10ReduceOneStep fuzzes the same rule over a 1e69-wide window above each threshold, checked against LibExactFloat.mul's schoolbook 512-bit product.

QA

• Discriminating tests: testLog10ReduceOneStepExamples, testLog10ReduceOneStep. On 886ca07 the suite left 12 mutants surviving (reciprocal -1, reciprocal +1 and borrow dropped, for each of steps 1-4). This PR's tests kill all 12, verified by mutation-probe with --only.
• Mutations applied: 32 in steps 1-4 (threshold +-1, condition negated, reciprocal +-1, borrow dropped, seed increment +1, quotient discarded). The existing testLog10ReduceAtEachThreshold, testLog10ReduceBelowEachThreshold and testLog10ReduceTop kill the threshold, condition and seed mutants. testLog10ReduceOneStepExamples kills reciprocal +-1 and borrow dropped. testLog10ReduceOutOfRange killed the range check (R01-R04) in an earlier run.
• Oracle: floor(x K / 2^256), with K = 2^256 / 10^(2^-i) rounded to nearest as the NatSpec documents. K and the expected quotients come from bc, independent of the library, and the fuzz test uses LibExactFloat's schoolbook 512-bit multiply.
• Category check: test-only mutation coverage for log10Reduce lines 1 of 3, with no issue. Every mutant in scope is now killed, and there is no src change.

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[linear]

RAI-3241

[github-actions]

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