Networking update

What changed

Added concurrency caps for /get-checkpoint and /submit-checkpoint so transfers stop crushing each other during bursty download/upload windows.
Validators now start evaluating miner submissions during the Submission phase instead of waiting for Validate, which gives slow-but-valid submissions a better chance to be processed in-cycle.
Retuned cycle timing to support that flow: longer Submission window, shorter Validate window.
CI now builds validator images for PRs targeting master too, not just staging, so Docker breakages get caught earlier.

What to expect

Validators: if you’re running Watchtower, these changes should be pulled in automatically.
Miners: no update is needed on your side.
Miners should start seeing improvement as early as the upcoming cycle — submissions should be easier and more reliable.
Distribution may still be slow in some cases. We’re continuing to work on distribution-side improvements tomorrow.