Add multi-dimensional integration (Phase 3)

[gvonness-apolitical]

Summary

• Tensor product rules: TensorProductRule with mixed-order and isotropic constructors, CubatureRule flat node storage with integrate() and integrate_box()
• Smolyak sparse grids: SparseGrid::clenshaw_curtis(dim, level) using combination technique with quantised point merging
• Genz-Malik adaptive cubature: degree-7/5 embedded rule pair, h-adaptive BinaryHeap subdivision, fourth-difference split axis selection, 1D delegation
• Monte Carlo / quasi-Monte Carlo: Sobol (gray-code, 40 dims, Joe-Kuo direction numbers), Halton (100 primes), plain MC (Xoshiro256++ PRNG, Welford variance)

136 unit tests + 28 doc tests, zero clippy warnings.

Test plan

• cargo test — 164 tests pass
• cargo clippy -- -W clippy::all — zero warnings
• cargo fmt --check — clean