function lookAlong
cinematic.ts:672
The orientation whose −Z is forward and whose +Y leans toward upHint.
Q.fromUnitVectors alone leaves the roll wherever the shortest arc dropped
it, which for a composition is the one degree of freedom that matters most —
a tilted horizon reads as an error before anything else in the frame does.
Here rather than in devtools/shots.ts (which re-exports it) because the
cinematic evaluator needs it and the layering only permits the import in
this direction.
ReturnsQuat