feat(governance): wallet_drep_vote_cast + pallas voting_procedures patch

Phase 6, key-credentialed slice (script-DRep bridge for the DAO is the
remaining sub-arc).

## pallas-fork patch (Sulkta-Coop/pallas feat-aux-data HEAD 507fd9da)

Threads voting_procedures through StagingTransaction → conway::
build_conway_raw, mirroring the auxiliary_data + certificates patches.

- pallas-txbuilder/src/transaction/model.rs: voting_procedures field +
  builder methods .voting_procedures() / .clear_voting_procedures()
- pallas-txbuilder/src/conway.rs: VotingProcedures::decode_fragment on
  the way out, assigned to TransactionBody.voting_procedures
- BRANCH-NOTES.md: section 3 added documenting the new patch
- 2 new tests (round-trip + negative path) on the txbuilder side

aldabra Cargo.lock SHAs bumped to the new HEAD.

## aldabra-core/src/governance.rs

- VoteChoice enum (Yes/No/Abstain) with into_pallas() conversion
- build_signed_drep_vote_cast — assembles VotingProcedures CBOR
  (NonEmptyKeyValuePairs<Voter, NonEmptyKeyValuePairs<GovActionId,
  VotingProcedure>>) with this wallet's stake credential as a
  Voter::DRepKey, attaches via the new pallas API, dual-witness signs.
- Optional CIP-100 anchor on the vote.

## aldabra-mcp/src/tools.rs

- wallet_drep_vote_cast tool: gov_action_tx_hash + gov_action_index +
  vote (yes/no/abstain) + optional anchor.

What's still scope-of-Phase-6:
- Script-credentialed DRep voting (the DAO governor as DRep, with
  redeemer-driven authorization). Needs a different signing path
  since the voter is a script credential, not a key credential.
  Separate builder; defer until Sulkta wants to actually bridge.

crates/aldabra-core/src/lib.rs

@@ -65,8 +65,8 @@ pub use plutus::{
 pub use stake::{build_signed_stake_delegation, parse_pool_id, STAKE_KEY_DEPOSIT_LOVELACE};
 pub use governance::{
     build_signed_drep_deregistration, build_signed_drep_registration,
-    build_signed_vote_delegation, parse_drep_target, DRepTarget,
-    DREP_REGISTRATION_DEPOSIT_LOVELACE,
+    build_signed_drep_vote_cast, build_signed_vote_delegation, parse_drep_target,
+    DRepTarget, VoteChoice, DREP_REGISTRATION_DEPOSIT_LOVELACE,
 };
 pub use tx::{
     build_signed_payment, build_signed_payment_with_assets, build_unsigned_payment,