pallas-txbuilder: thread voting_procedures through StagingTransaction → Conway build

Fills the third of pallas-txbuilder's Conway TODOs (after auxiliary_data
+ certificates):

- StagingTransaction gains `voting_procedures: Option<Vec<u8>>` (opaque
  CBOR — same Eq-derive workaround as auxiliary_data + certificates)
- Builder methods .voting_procedures(cbor_bytes) + .clear_voting_procedures()
- conway::build_conway_raw decodes via VotingProcedures::decode_fragment
  and assigns to TransactionBody.voting_procedures

VotingProcedures is itself a NonEmptyKeyValuePairs<Voter,
NonEmptyKeyValuePairs<GovActionId, VotingProcedure>> map, so callers
pre-assemble the full map and encode once.

Tests:
- voting_procedures_plumb_through_to_tx_body — encode DRepKey vote-Yes
  on a synthetic GovActionId, build, decode, assert round-trip
- no_voting_procedures_means_none — negative path

BRANCH-NOTES.md

@@ -57,7 +57,32 @@ Tests added:
   asserts the cert round-trips byte-for-byte.
 - `no_certificates_means_none` — negative path.
 
-### 3. `pallas-addresses` — `pub fn new` on `StakeAddress`
+### 3. `pallas-txbuilder` — `voting_procedures` field on `StagingTransaction`
+
+Upstream had `voting_procedures: None, // TODO` in the conway builder.
+Without this, **DRep / SPO / committee voting on Conway governance
+actions can't ride a tx built with pallas-txbuilder.**
+
+Added 2026-05-06:
+- `pub voting_procedures: Option<Vec<u8>>` field (opaque CBOR for the
+  same `Eq`-derive reason as `auxiliary_data`).
+- Builder methods `.voting_procedures(cbor_bytes)` and
+  `.clear_voting_procedures()`.
+- `conway::build_conway_raw` decodes via `VotingProcedures::decode_fragment`
+  and assigns to `TransactionBody.voting_procedures`.
+
+The `VotingProcedures` type is itself a single map
+(`NonEmptyKeyValuePairs<Voter, NonEmptyKeyValuePairs<GovActionId,
+VotingProcedure>>`), so callers pre-assemble the full map and encode
+once. No staged accumulator needed.
+
+Tests added:
+- `voting_procedures_plumb_through_to_tx_body` — encodes a
+  single DRepKey-vote-Yes on a synthetic GovActionId, builds, decodes,
+  asserts the vote round-trips byte-for-byte.
+- `no_voting_procedures_means_none` — negative path.
+
+### 4. `pallas-addresses` — `pub fn new` on `StakeAddress`
 
 Upstream defined `pub struct StakeAddress(Network, StakePayload)` with
 **unexported tuple fields** and no `new()` constructor — so external
@@ -103,6 +128,7 @@ per change) for upstream review ergonomics:
 
 - `pallas-txbuilder: thread auxiliary_data through StagingTransaction → Conway build`
 - `pallas-txbuilder: thread certificates through StagingTransaction → Conway build`
+- `pallas-txbuilder: thread voting_procedures through StagingTransaction → Conway build`
 - `pallas-addresses: pub fn new on StakeAddress`
 
 ## Change discipline going forward