feat(univariate): randomized Cantor-Zassenhaus root search

[olympichek]

Overview

This PR adds a Las Vegas randomized Cantor-Zassenhaus univariate root-search backend.

This PR includes:

• A configurable randomized splitter with probe families and deterministic fallback behavior.
• Odd-characteristic splitting and characteristic-two trace splitting to support both odd and even (binary) fields.
• Soundness and completeness proofs for the Las Vegas root-search.
• Probabilistic proofs for the randomized splitter: each split attempt succeeds with probability at least 1/2 under uniform probes
• Regression tests for Las Vegas root search.
• Benchmarks comparing different univariate root search backends.

Technical details

The source of randomness in the executable code is caller-supplied ProbeFamily data. However, once the probe family, config, and input polynomial are fixed, the splitter is pure and deterministic. Tests and concrete field contexts use deterministic probe families; the binary benchmarks precompute fixed-seed probe tables before timing and then pass those tables through the same explicit probe interface.