Variable Secp256K1Sha256TRConst

FROST(secp256k1, SHA-256) with Taproot ciphersuite.

This ciphersuite is compatible with BIP-340 (Schnorr) and BIP-341 (Taproot). Key differences from standard FROST(secp256k1, SHA-256):

• Context string: "FROST-secp256k1-SHA256-TR-v1"
• H2 uses BIP-340 tagged hash "BIP0340/challenge"
• 64-byte signature serialization (x-only R point, no prefix)
• Public keys are ensured to have even Y coordinate
• Supports Taproot key tweaking