The pull's running-check line updates live and counts files and tests from the shard runner's own lines #2280

Second half of #1123.

Done looks like:

• [ ] The checks line is a data-live-src region, like /live/apps, so it updates without a reload.
• [ ] While the check runs, it adds "61 of 170 files · 812 passed, 0 failed". The counts come only from lines the shard runner itself writes. When no count is known yet, the count is left out rather than guessed.
• [ ] Test: the live fragment's HTML changes when the run's counts change.