dblock / excalc-rs / 35948400238

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Implement number theory functions (gcd, lcm, fib, primes, Mersenne, perfect, Fermat, sigma/tau/phi)

Adds src/functions/number_theory.rs with a deterministic Miller-Rabin
primality test (valid for the full u64 range) backing gcd, lcm, fib,
isprime, moebius, mersenne, perfect, fermat, safeprime, primec, primen,
mersennegen, mersgen, genmers, sigma, tau, and phi/eind.

Notable clarifications resolved during implementation (see docs/functions/number-theory.md
for details):
- mersenne(p) is the literal 2^p - 1 for exponent p (not an upper bound
  on the result), despite an inconsistent worked example in the
  original manual.
- mersennegen/mersgen/genmers search generators in [2, 63] via
  primality testing rather than a hardcoded list of known Mersenne
  primes.
- fermat(k) is computed generically via the 2^(2^k)+1 formula for any
  k in [0, 9], not just the historically known-prime k <= 4.
- primec/primen's "1 counts as prime" note in the pre-existing docs
  didn't match the worked examples; implemented to match the examples
  instead (primec doesn't count 1; primen(1) is a special-cased
  override of the otherwise-conventional 1-indexed prime enumeration).

Wires these into eval.rs::call_function, moves
docs/functions/number-theory.md from planned to v1, updates DESIGN.md's
scope/catalog, adds a README Examples block (also exercises eval.rs's
dispatch arms via tests/readme_examples.rs), and updates CHANGELOG.md.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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