Mutation coverage: IMPL-div-extremes

[thedavidmeister]

Mutation campaign over the extreme branches of LibDecimalFloatImplementation.div at 886ca07: the exponentA floor clamp, the exponentB transfer, underflowExponentBy, the ExponentOverflow mirror, the 2^255 quotient revert, and the u > 76 and quotient-zero returns. 27 mutants, probed with mutation-probe against LibDecimalFloatImplementationDivTest, LibDecimalFloatImplementationMainEquivalenceTest and LibDecimalFloatDivTest.

Before this PR: 24 killed and 3 survived. All 3 survivors are equivalent:

• M06 and M08 change the spill-overflow return. On that path exponentA is type(int256).min, so any exponentB above 76 gives u > 76 and the result is (0, 0) anyway.
• M24 moves the zero-return threshold from u > 76 to u > 77. At u = 77, 10^77 < 2^256 and |q| <= 2^255 < 1e77, so q / 10^77 is 0 and the quotient-zero return gives (0, 0).

Changes:

• testMainEquivalenceDivFloorExamples failed with MemoryOOG on unmodified 886ca07. It is also red in main's rainix-sol run 38170850977. Memory grew across the 76 test: equivalence-only coverage for implementation div, add, sub and inv #368 sheds in one call frame. Each shed now runs in its own external call, so the test passes at 184M gas and covers the u = 1..76 sheds deterministically again. It now kills M23 and M25.
• Before this PR, only the testDivByOneAtFloor fuzz run killed the signedCoefficientA == type(int256).min conjunct of the 2^255 revert (M18). The new testDivNegativePowerOfTenDivisorAtMax kills it deterministically. Expected values: 1e76 10^(max-76) / (-1 10^-76) is exactly -1e76 10^max, and the same holds for the other signs and for type(int256).max.

Adversarial pass: a grid of 14 coefficients by 17 exponents for each operand, 56644 cases at the int256 extremes, compared against LibExactFloat.divPartsWide and isTruncatedQuotient. Every case matched. No bug candidates.

QA

Discriminating tests: testDivNegativePowerOfTenDivisorAtMax (kills M18 deterministically); testMainEquivalenceDivFloorExamples, repaired from MemoryOOG (kills M23 and M25)
Mutations applied: 27 mutants via mutation-probe, with forge test --mc limited to the three listed contracts. Pre-existing suite: 24 killed, 3 equivalent (M06, M08, M24). After this PR: M18, M23 and M25 are also killed by the tests above.
Oracle: exact maths of the NatSpec rule (a / b truncated toward zero at 10^(ua - lb), floor shed, 2^255 shed, ExponentOverflow above int256.max), as encoded by test/lib/LibExactFloat.sol and by the fixed examples' exact products
Category check: test-only. No src changes. One baseline-broken test repaired without weakening its assertions.

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• test/src/lib/implementation/LibDecimalFloatImplementation.div.t.sol
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[linear]

RAI-3237