RFC: Post-quantum signature experimentation tooling for contrib/ #36424

Problem

Bitcoin's ECDSA (secp256k1) and Schnorr signatures are vulnerable to a cryptographically relevant quantum computer via Shor's algorithm. Per BIP-361, migration requires experimenting with post-quantum output types.

Proposal

Add an additive, self-contained experimental toolset under contrib/ (e.g. contrib/pq-shield) that:

• generates ML-DSA-65 (FIPS 204, NIST level 3) keypairs,
• signs and verifies messages as a CLI,
• ships a small audit utility validating key sizes and file permissions,
• is covered by unit tests and is not wired into the main build.

Scope is strictly limited to contrib/; no consensus, validation, or networking changes are proposed. The goal is to give developers and wallet authors a reference implementation to evaluate PQ signature sizes, performance, and integration surfaces before any consensus-level BIP discussion.

Open questions

• Should such tooling live in-tree or as a separate depends-style submodule?
• Is liboqs an acceptable external dependency for contrib tools?

Feedback welcome before any PR is opened.

This should be posted on the bitcoin-dev mailing list, the Delving Bitcoin forum or some other platform where broad, protocol-level concepts are discussed. Conceptual questions and most usage questions can be posted on Stack Exchange. The Bitcoin Core issue tracker is reserved for discussion about this specific software project only, its implementation and usage.