feat(lez-multisig): batch_execute instruction for coordinator voting flow

[jimmy-claw]

Problem

Current LP-0002 private multisig requires N+2 on-chain transactions (propose + N approve txs + execute). For coordinator-based workflows, this is inefficient — the coordinator should be able to collect ZK proofs off-chain and submit everything in a single transaction.

Repo: jimmy-claw/lez-multisig (jimmy/private-multisig-v2 branch)
Related: #87 (SPEL dry-run output format for coordinator interop)

Proposed Solution

Add a batch_execute instruction in lez-multisig that accepts all vote proofs inline:

#[instruction]
pub fn batch_execute(
    multisig_state: AccountWithMetadata,
    proposal_index: u64,
    create_key: [u8; 32],
    /// (vote_receipt, nullifier) pairs — collected off-chain from each member via `spel approve --dry-run`
    approvals: Vec<(Vec<u32>, [u8; 32])>,
    members: Vec<NullifierPublicKey>,
    chained_call: ChainedCall,
) -> LezResult<()> {
    // 1. Verify each (receipt, nullifier) pair against membership
    // 2. Check nullifier uniqueness
    // 3. Count approvals >= threshold
    // 4. If met: fire ChainedCall
}

Coordinator Workflow

# Each member generates their proof blob (no on-chain submission)
spel approve --caller <private_key> --dry-run > member1_proof.txt
spel approve --caller <private_key> --dry-run > member2_proof.txt

# Coordinator collects files (out of band) and submits in one tx
spel batch_execute --approvals member1_proof.txt,member2_proof.txt ...

Design Notes

• Same vote circuit binary — each (receipt, nullifier) verified independently
• NSKs never leave clients — coordinator only receives proof blobs
• Privacy guarantees identical to sequential approve flow
• approve kept for async/online voting
• execute kept for stored-approvals case

Acceptance Criteria

• batch_execute instruction in lez-multisig
• ZK proof verification per approval entry
• Nullifier uniqueness check
• Threshold enforcement
• ChainedCall execution on success
• IDL auto-generated via #[lez_program] macro
• E2E demo with coordinator/batch_execute flow
• SPEL confirms --dry-run output format is stable (docs: confirm #[pre_tx_hook] dry-run output format is stable for coordinator interop #87)