Add Pell equation benchmark

[alcides]

Adds a Pell equation benchmark for the positive solutions of x^2 - 2y^2 = 1.

• Minimizes x + y after enforcing the equation
• Uses (x, y) = (3, 2) as the optimum
• Adds a standard example entry
• Registers it in the benchmark suite and run_examples.sh
• Adds tests

Signed integer solutions would make the sum unbounded below, so this benchmark uses the conventional positive Pell solutions.

Validation: uv run pytest tests/benchmarks/test_mathematical.py -q (6 passed); Ruff passes.