Professionalize multilinear Sumcheck research repository

[qoosmo]

Summary

Professionalizes the repository for external research and hiring review.

Project identity

• renames the project/package to multilinear-sumcheck
• renames the Rust library target to multilinear_sumcheck
• corrects repository metadata and description

Documentation

• adds a research-grade README
• documents the algorithmic structure and trust boundary
• adds reproducible benchmark guidance
• adds a minimal runnable Sumcheck example
• adds contribution and security guidance
• adds dual MIT / Apache-2.0 license files

Correctness / robustness

• handles the zero-variable Sumcheck verifier case without panics
• adds zero-variable verification tests
• normalizes crate references after the rename
• fixes the benchmark-group documentation mismatch

Quality gates

• cargo fmt --all
• cargo check --all-targets --all-features
• cargo test --all-features
• cargo clippy --all-targets --all-features
• cargo bench --bench sumcheck --no-run

The cryptographic scope remains intentionally narrow: this is the algebraic Sumcheck core, not a complete SNARK/STARK. The README explicitly documents the missing Fiat-Shamir, commitment, zero-knowledge, recursion, and production-hardening layers.