fix(core): return error instead of panicking on negative power edge cases 💥

[timfennis]

Context

While fuzzing the pipeline for panics, I found that the integer-power (^) path aborts the whole process on several negative-exponent inputs. These are recoverable runtime errors that should never crash the interpreter — same class of bug as the recent integer division-by-zero fix (#168).

The crashes

Input	Panic
2 ^ (1/-1)	right hand side must not be negative (int.rs:117)
0 ^ -1	denominator == 0 (num-rational)
0 ^ (2/-1)	right hand side must not be negative

Root cause — Number::pow had two integer-exponent branches:

• Int ^ Int handled negative exponents as the reciprocal 1/(base^|exp|), but with base 0 that builds BigRational::new(1, 0), which panics.
• Int ^ Rational (integer-valued rational, e.g. 1/-1) didn't handle negatives at all — it called Int::pow directly, which panics unconditionally on a negative RHS.

The existing MAX_EXPONENT_BITS guard only checks magnitude, so it never caught these.

Changes

• Added a Number::int_pow(base, exponent) helper that both integer-exponent branches route through. It returns a division by zero error for 0 ^ negative, computes the reciprocal rational for any other negative exponent, and otherwise returns the plain integer power. This also removes the previously duplicated negative-exponent logic.
• Regression tests:
  • 900_bugs/bug0025_pow_negative_rational_exponent.ndc — 2 ^ (1/-1) → 1/2
  • 001_math/030_pow_zero_negative_exponent.ndc — 0 ^ -1 → division by zero
  • 001_math/031_pow_zero_negative_rational_exponent.ndc — 0 ^ (2/-1) → division by zero

Valid cases verified unchanged: 2 ^ -1 → 1/2, 3 ^ (-4/2) → 1/9, 2 ^ 3 → 8, (1/2) ^ -3 → 8.

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