[Draft-Extensible-Tx]: Add digest algorithms for txid, signature, and auth commitment.

Specifies the V6 transaction digest algorithms:
- TxId digest with separate value_pool_deltas_digest and dynamic effects_bundles_digest
- Signature digest for transparent input signing with hash_type support
- Authorizing data commitment for witness data

Key design decisions:
- Value balances committed via top-level value_pool_deltas_digest (not per-bundle)
- effects_bundles_digest uses tagged concatenation (bundle_type_id || root_hash)
- Unknown bundle types supported by accepting their root hash externally

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: nuttycom