feat: merkle batch payment external signer support

[Nic-dorman]

Summary

Extends the external signer support (PR #10) to cover merkle batch payments — the gas-efficient path for uploads with 64+ chunks.

The problem: pay_for_merkle_batch requires a Wallet on the Rust side. Desktop apps using WalletConnect can't call wallet.pay_for_merkle_tree() directly.

The solution: Same prepare/finalize pattern as the wave-batch PR:

1. client.prepare_merkle_batch_external(addresses, data_type, data_size) — builds tree, collects candidate pools → returns PreparedMerkleBatch with depth, pool_commitments, timestamp
2. Frontend calls pay_for_merkle_tree(depth, pool_commitments, timestamp) via WalletConnect → gets back winner_pool_hash
3. finalize_merkle_batch(prepared, winner_pool_hash) — finds winner pool, generates per-chunk proofs

Changes

• PreparedMerkleBatch: New struct with public payment params + private tree/pool data
• prepare_merkle_batch_external(): Phase 1 — no wallet needed
• finalize_merkle_batch(): Phase 2 — no wallet needed
• ExternalPaymentInfo enum: WaveBatch(PaymentIntent) | Merkle(PreparedMerkleBatch) — file_prepare_upload auto-selects based on chunk count
• finalize_upload_merkle(): Handles merkle variant
• Refactored pay_for_merkle_single_batch: Now uses prepare + finalize internally

No breaking changes

Existing wallet-based flows (file_upload_with_mode, pay_for_merkle_batch) work identically — pay_for_merkle_single_batch was refactored to use the new functions internally.

Tests

• 3 new unit tests for finalize_merkle_batch (valid winner, invalid winner, proof deserialization)
• All 108 tests pass (105 existing + 3 new)
• Clippy clean, fmt clean

Test plan

• cargo check --all passes
• cargo clippy -p ant-core -- -D warnings clean
• cargo fmt --all -- --check clean
• cargo test -p ant-core --lib — 108 tests pass
• E2E: prepare_merkle_batch_external → external payment → finalize_merkle_batch on devnet

Usage from ant-gui

let prepared = client.file_prepare_upload(&path).await?;

match prepared.payment_info {
    ExternalPaymentInfo::WaveBatch(intent) => {
        // Frontend calls payForQuotes() with intent.payments
    }
    ExternalPaymentInfo::Merkle(ref merkle) => {
        // Frontend calls pay_for_merkle_tree(depth, pool_commitments, timestamp)
        // Returns winner_pool_hash
    }
}

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