R1:f5c7dd68d217

Clean up some ordering and strata edge cases in Phrequent

Summary:
Ref T3569. Two issues:

# Since `sort()` is not stable, instantaneous events (ending on the same second they start) would sometime sort wrong and produce the wrong results. Guarantee they sort correctly.
# Because events can end at any time, there are some additional special cases the algorithm didn't handle properly. Draw a bunch of ASCII art diagrams so these cases work properly.

Test Plan:
- No more fatal when tracking an object…