Add KZH-k multilinear polynomial commitments

[h-hafezi]

Description

This PR adds KZH-k, a non-hiding pairing-based multilinear polynomial
commitment family parameterized by the const generic K.

The implementation follows KZH-Fold and uses the generic-opening auxiliary
commitments described by IronDict. It uses arkworks' native little-endian MLE
layout throughout and calls MultilinearExtension::fix_variables for partial
evaluation.

Highlights

• Implements the native PolynomialCommitment interface for dense and sparse
  arkworks multilinear extensions.
• Supports balanced rectangular tensor decompositions when the number of
  variables is not divisible by K.
• Stores only auxiliary commitment tables that are beneficial to the generic
  opening algorithm.
• Supports same-point batching, the framework's default multi-point and
  linear-combination APIs, and prepared verifier keys.
• Validates family markers, tensor dimensions, keys, commitments, states,
  proofs, labels, and point dimensions at operation boundaries.
• Includes canonical serialization tests, independent tensor-order oracles,
  malformed-input tests, cost-bound tests, and a KZH benchmark covering
  K = 2, 3, 4, 6 and 12, 16, and 20 variables.

For N = 2^n evaluations, the balanced construction has:

• at most O(K * N) commitment preprocessing work;
• O(N^(ceil(K / 2) / K)) online group work plus O(N) field work for a
  generic opening, giving O(sqrt(N)) group work for even K;
• O(K * N^(1/K)) proof size and verifier work.

This PR intentionally does not add hiding commitments, strict degree bounds,
a Boolean-specialized opening API, or an R1CS verification gadget.

The most important files to review are:

• poly-commit/src/kzh/mod.rs
• poly-commit/src/kzh/data_structures.rs
• poly-commit/src/kzh/utils.rs
• poly-commit/src/kzh/tests.rs

No existing GitHub issue currently tracks this addition; the construction,
scope, costs, setup assumptions, and compatibility choices are described here
and in the module documentation.

Validation

• cargo test -p ark-poly-commit kzh::tests:: (31 passed)
• cargo test -p ark-poly-commit kzh::tests:: --no-default-features --features std (30 passed)
• cargo test --workspace --all-features (145 passed)
• cargo check --workspace --all-targets --all-features
• cargo check -p ark-poly-commit --no-default-features
• cargo fmt --all -- --check
• cargo doc -p ark-poly-commit --no-deps --all-features

---

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work. (Design and scope are explained above.)
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer