Key Derivation for Single Key P2TR Outputs.

Sets the key layout wallets use for single-key Taproot addresses, so they can be found again from one backup.

Many wallets and signing devices back up only the seed, not which kinds of address were used. This BIP reserves branch 86 of the key tree for Taproot addresses controlled by one key, following BIPs 49 and 84, so a wallet restoring the backup is likely to find those coins.