Replace the local voting-key filter with the upstream rust-dashcore helper once #916 lands

[bfoss765]

Why the local copy exists

PR #4258 surfaces a "which masternodes does this voting key control?" lookup to
Kotlin, for contested-username voting after the dashj engine is held
post-cutover. It needs rust-dashcore's
MasternodeList::masternodes_by_voting_key, which is not on the Dash-owned
rust-dashcore revision this workspace pins (70d4bf8e36057c58e02d56769a6e9760f701dd06).

The helper does exist upstream, but only as the last of four commits on
dashpay/rust-dashcore#916 (branch feat/owner-tagged-reservations) — a PR whose
main subject is owner-tagged reservations, and which is currently
CHANGES_REQUESTED, conflicting against dev, and whose reviewer has asked for
it to be split. Waiting on it is therefore open-ended.

Pinning the workspace to a revision that carries the helper meant redirecting
all eight rust-dashcore workspace dependencies at a personal fork
(bfoss765/rust-dashcore), which also dragged in ~528 lines of unrelated
owner-tagged-reservation and asset-lock behaviour. That fork pin was rejected in
review on #4258. The workspace therefore stays on the Dash-owned revision and
#4258 carries a small local copy of the filter instead.

The filter reads only long-standing public SML fields (key_id_voting,
pro_reg_tx_hash), so the local copy is low-risk — but it is duplication, and
this issue exists so it does not become permanent.

The duplication site

• File: packages/rs-platform-wallet/src/spv/runtime.rs
• Function to delete: private free fn
  masternodes_by_voting_key(list: &MasternodeList, voting_key_id: &PubkeyHash) -> Vec<[u8; 32]>
• Its tests: #[cfg(test)] mod masternodes_by_voting_key_tests in the same file
• Sole caller: SpvRuntime::masternodes_by_voting_key_blocking

The local function folds the ProTxHash -> [u8; 32] (internal byte order)
conversion into the same pass; the upstream helper returns Vec<ProTxHash>, so
the conversion has to come back when swapping.

What to do once the helper lands upstream

1. Move the workspace rust-dashcore pin in Cargo.toml (all 8 deps) and
   Cargo.lock to a Dash-owned revision that contains
   MasternodeList::masternodes_by_voting_key.
2. Delete the private masternodes_by_voting_key free function in
   runtime.rs.
3. In SpvRuntime::masternodes_by_voting_key_blocking, replace
   masternodes_by_voting_key(list, voting_key_id) with the upstream call plus
   the byte-order conversion:

   list.masternodes_by_voting_key(voting_key_id)
       .into_iter()
       .map(|pro_tx| {
           let mut out = [0u8; 32];
           out.copy_from_slice(pro_tx.as_ref());
           out
       })
       .collect()

4. Delete mod masternodes_by_voting_key_tests, or keep it as a regression test
   against the upstream helper — it only touches public API, so it compiles
   either way.
5. Drop the use dashcore::sml::masternode_list::MasternodeList; import from
   runtime.rs if nothing else there needs it.

Links

• Upstream helper PR: feat(key-wallet): owner-tagged reservations to close the broadcast-release TOCTOU (platform#4185) rust-dashcore#916 — helper is commit 4d927c15
  (feat(sml): masternodes_by_voting_key lookup on MasternodeList)
• Consuming PR: feat(sdk): masternodes-by-voting-key lookup (contested-username voting post-cutover) #4258