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.

docs/CURRENT-STATE.md

@@ -427,6 +427,8 @@ reference. Legacy projects retain their ordinary flights, content and edits.
 Seeking between ordinary camera poses uses the shortest longitude arc across the
 dateline while preserving both authored endpoint longitudes, and turns heading
 and roll the short way for any authored angles.
+A seek to a shot's start, as a shot action makes, selects that shot rather than
+the end of the previous one.
 See [camera directions](DIRECTOR-CAMERA.md).
 
 Realtime voice composes separate connection, response/tool, Radio handoff, input