[add] Observe UTXO Outbounds Immediately After Signing

Resolves #1559 (closed). Seeking insight from @heimdallthor / @son-of-odin that the approach is sane - I did not see an obvious way to do this without updating the interface to allow the chain client to return the observation after signing, since they have agency to modify the amount and gas.

Side note: this required some fiddling with the times for smoke test to avoid the raciness from the lower latency outbound observations. (disabled in smoke tests instead - see comment below)