This PR proposes NestedMuSig2 for BIP340-compatible Signatures, a BIP draft extending BIP327 MuSig2 to support trees of nested signing groups. Each subgroup participates through its aggregate public key, and the complete tree produces a single BIP340 signature verifiable without knowledge of the internal structure.
The proposal includes the specification, a Python reference implementation, and test vectors covering nested key and nonce aggregation, signing, partial-signature verification, signature aggregation, and error handling. Multi-level examples and randomized tree tests accompany the vectors.
Review is particularly welcome on recursive nonce binding, signer session construction, root tweaks, and compatibility with BIP327 in the flat case. The construction remains n-of-n at each aggregation node.
