docs(ci): verify Qt cache reuse and visualize libobs build trials

[zackees]

Summary

Refs #1286.

• Verify the docs-only Qt consumer restored the producer's exact 1,161,888,219-byte archive, hit 298/300 compilations, passed all 15 Qt tests, and skipped publishing a duplicate Qt snapshot.
• Add the reproducible isolated libobs timing chart with all three measured observations and explicit revision/attribution limits.
• Preserve the proposed recording-wait patch and controlled reproduction as evidence offered to the existing frontend: RecordingShutdown polls every 10 ms on the UI thread while waiting for recording to stop #1283 claimant. No production helper or existing assertions change.

Validation

Documentation/evidence only: no artificial RED/GREEN test. Parsed all three updated evidence JSON files and the SVG successfully. Chart derives from already published raw build trials; initial warm seed excluded. No workflow, product source, submodule pointer, or cache writer policy changes.

The consumer's build/test duration was 74 seconds versus 54 seconds for the producer; unmatched runners do not prove a speedup. The demonstrated benefit is avoiding another Qt archive upload. Remaining Ubuntu-compatible reuse, representative-day results, and #1362 integrity blocker are not declared complete.