Bump to forked vdf after modernization

[jose-compu]

Context

chaincraft-rust should consume the forked vdf repository after the fork is modernized to current stable Rust.

Goal

Track and execute the dependency bump in chaincraft-rust once jose-compu/vdf modernization is complete.

Proposed scope

• Update vdf dependency source/reference to the forked modernized repo.
• Adjust API integration points if modernization introduces breaking changes.
• Run full verification (cargo check, cargo test, cargo clippy).
• Document the dependency switch and any migration notes in README/release notes.

Blocked by

• Modernization issue in jose-compu/vdf (stable Rust compatibility + CI green).

Acceptance criteria

• chaincraft-rust builds/tests cleanly with forked vdf.
• CI remains green.
• Dependency source is pinned/documented per project policy.