Add Shamir secret sharing helpers for scalar polynomials

[LLFourn]

Summary

This PR makes creating and restoring secret backups easier by adding Shamir secret sharing functionality to the polynomial module.

Changes

• Add generate_shamir_sharing_poly to create polynomial with secret at x=0
• Add trusted_dealer_shamir_sharing that returns an iterator of shares
• Update interpolate_and_eval_poly_at_0 docs to clarify it reconstructs secrets
• Add comprehensive tests including proptest for random testing

Example Usage

use secp256kfun::{poly, prelude::*};
use rand::seq::SliceRandom;

let my_secret = s\!(42);
let shares: Vec<_> = poly::scalar::trusted_dealer_shamir_sharing(my_secret, 3, 5, &mut rand::thread_rng()).collect();

// Sample 3 random shares (threshold amount) to reconstruct the secret
let mut rng = rand::thread_rng();
let random_shares: Vec<_> = shares.choose_multiple(&mut rng, 3).cloned().collect();

let recovered_secret = poly::scalar::interpolate_and_eval_poly_at_0(&random_shares);
assert_eq\!(recovered_secret, my_secret);

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Abridged conversation transcript

Human: Ok can you review the currently staged changes.

Assistant: [Reviews staged changes showing new Shamir secret sharing functions: generate_shamir_sharing_poly, trusted_dealer_shamir_sharing, and updated docs for interpolate_and_eval_poly_at_0]

Human: I think from trusted_dealer_shamir_sharing we can just return an impl Iterator

Assistant: You're right! [Modifies function to return iterator instead of Vec]

Human: cool before making a PR to fix that can you make a quick PR to fix that format warning. about format_code_in_doc_comments.

Assistant: [Creates PR #213 to remove format_code_in_doc_comments from rustfmt.toml]

Human: Before we do that can you make a proptest that tests doing trusted_dealer_shamir_sharing and then reconstruction.

Assistant: [Creates comprehensive proptest in tests/poly.rs that tests reconstruction with random secrets, various thresholds, and different numbers of shares]

Human: oh I wonder what the type parameter for trusted dealer sharing is actually there for.

Assistant: [Examines code and finds unused type parameter Z: ZeroChoice that was declared but never used in the function]

Human: I think remove it

Assistant: [Removes unused type parameter from function signature and updates tests]

Human: cool. Make a PR. Attach a summary of this conversation to it. Note at the beginning that the purpose was to make creating and restoring secret backups easier to work on.

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