Bring the branch current with upstream staging (= master + #748). The earlier
"Merge branch master into contracts" used a stale local master snapshot, which
left the branch ~41 commits behind; this merges the current tip, including the
grin 5.4.0 dependency bump (#745) and the tx-delete / retrieve_txs fixes.
Only Cargo.lock conflicted (regenerated). Full cargo test --workspace green
(52 binaries / 185 tests).