Surface execution timing in results

Neither `QCDLResult` nor its experiments carry any timing currently.

Populate timing from the cloud client's per-problem data (`Future` timing / problem info) -- per-experiment `time_taken` in the header and an aggregate `time_taken` on the `QCDLResult`. Note: `Result.date` is currently the result-assembly time, not the execution time; consider aligning it.

Start with QCDLResult and the experiment result structures, then trace the cloud client's Future/problem info to locate per-problem execution timing. Populate per-experiment and aggregate time_taken values, and evaluate whether Result.date should represent execution rather than assembly time. Done means results expose the requested timing consistently.

Written by the indexing model from the issue text.