fix(director): land on the requested shot

A shot action seeks to the target shot's startProgress, which
sceneTimingForShot computes as start / total. sceneSeekState multiplied
that back into seconds and compared it with each shot's end, but
(start / total) * total can round one ulp below start. The comparison
then put the seek at the very end of the previous shot. With five equal
3.9 s shots, "go to" shots 2, 3 and 5 landed on shots 1, 2 and 4.

Compare the requested progress against each shot's end / total instead,
the same division the caller used, so a shot's own start always selects
that shot. Exact boundaries still select the next shot, and anything
below a start still selects the previous one.

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+- Director shot actions land on the requested shot. A seek to a shot's start
+  could round a hair below it and select the end of the previous shot for some
+  shot counts and durations.
+
 - Add Barcelona's bike share to the GBFS registry: Bicing (543 stations, the
   city's PBSC system) and AMBici (237 e-bike stations across the metropolitan
   area, run by nextbike). Both are keyless GBFS 2.3 feeds listed in