Annex is ignored before signing

While preparing the transaction to be signed, annex extractor function expects multiple elements in the stack (such as sig + annex). This is a valid check after the tx is hashed and signed, but the same function also used while hashing the transaction before signing. This causes ignored annex before signing, causing an error when annex is included into witness later.

https://github.com/cmdruid/tapscript/blob/10459abc7d50e03c61a10c2d7b5416a36087232b/src/lib/sig/taproot/hash.ts#L207