Reconcile the contract feature with #755, which replaced the WalletBackend
trait with a concrete WalletBackend<C,K> struct (libwallet/src/backend.rs) and
removed impls/src/backends/lmdb.rs.
- Port the contract module and contract API entry points from the trait form
(T: WalletBackend<'a,C,K>) to the concrete struct.
- Re-home next_child_for and a parent_id+log_id get_tx_log_entry onto backend.rs.
- Re-apply the self-spend tx-log filters, cancel-loop, v5 payment-proof fields,
and Context contract fields onto staging's rewritten files.
- Adapt to fallible iter()/tx_log_iter() and self-consuming batch commit.
Full suite green (53 binaries / 191 tests).
jwinterm
3fba1319fb