A test bound read from the code under test lets runaway backoff pass

A virtual-clock test checked that every visible tile was either drawn, in flight, or had a timer due by the row's own slowUntil + 10 ms. A mutant that backed off forever (slowUntil + 1e9) still passed, because it moved the bound the test read. Fix: track how long each item has been unhandled and cap that gap with a constant the test owns (the documented max backoff + slack). Also run the invariant once after the virtual run ends, because once the timers run out the loop never calls it again. Related: when an adaptive estimator keeps the max of the samples (for example, the densest window seen), a single burst in a tiny sample pins it for good. Let a sample over a much wider span replace a narrower one, and size each next step from the step just measured.