fix: allow exact remainders with zero precision

[Shubham-Padkonde]

Fixes #134.

BaseContext.Rem(New(100, 100), New(10, 1)) currently returns DivisionImpossible even though the remainder is exactly zero. The quotient digit-count check treats the default precision of zero as a zero-digit limit, while the context uses zero to disable rounding.

Apply the quotient-size limit only when precision is nonzero, and document that behavior for Rem. Explicit precision limits and the existing special-value handling remain in place.

Regression coverage includes the reported large-exponent case, a large integer coefficient, fractional and negative operands, signed zero, and both result/operand aliases. A separate test checks the failing and successful sides of an explicit precision limit.

Validation on Windows amd64, Go 1.27.1:

• All 21 unlimited-precision regression subcases failed before the fix and pass afterward.
• go test -count=1 ./... passes.
• go vet -unsafeptr=false ./... passes.
• go test -run='^$' -bench=. -benchtime=1x ./... passes.
• git diff --check passes.

The Linux, race, older-Go, and ARM CI matrix has not been run locally.

Prepared with Codex assistance.