feat: add support for OP_RETURN output on the send path - #2753
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Adds an optional OP_RETURN (null-data) output to the send path. `zcash_client_backend` still plans and, for payload-free sends, builds the whole proposal. When a payload is present it rides the final transaction of the proposal (the ZIP-320 TEX exposure step, or the single step otherwise), which is built by driving the `zcash_primitives` transaction builder directly so `add_transparent_null_data_output` can be called; a Shield never carries a payload. Adds a validated `OpReturnData` newtype (<=80 bytes) with a typed error, and threads `Option<OpReturnData>` through `calculate_transactions`. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Replaces the sketch stub with a working THORChain / MAYAChain swap-deposit flow. `LightClient::propose_swap_deposit(vault, amount, memo, account)` sends two transactions, the same shape as a ZIP-320 TEX pair: 1. a deshield from shielded funds to a freshly reserved, wallet-owned transparent address, built entirely by zcash_client_backend, and 2. a transparent memo carrier spending that output to the vault with the swap memo in an 80-byte OP_RETURN, hand-built on the zcash_primitives builder (the only place OP_RETURN support exists upstream). The deshield funds the carrier exactly, so the carrier has no change and always balances; its transparent input is the refundable sender THORChain / MAYAChain require. Only the transparent carrier is hand-built; the shielded spend stays in zcb, and all consensus-critical pieces (builder, ZIP-317 fee, prover, persistence) are reused from upstream. New: wallet/op_return.rs (validated OpReturnData newtype), wallet/swap.rs (fee sizing, carrier build/persist, deshield-output location), the propose_swap_deposit orchestration, and typed errors. Tests: 9 offline unit tests plus a passing regtest integration test (swap_deposit_carries_op_return_to_vault_on_chain) proving both transactions confirm on-chain with the memo in the carrier's OP_RETURN. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Removes explanatory inline comments from the OP_RETURN swap-deposit code and its tests, leaving the doc comments. No behavior change. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Adds an optional OP_RETURN (null-data) output to the send path by keeping
zcash_client_backendfor planning and bypassingcreate_proposed_transactionsonly for the payload-carrying transaction.