Quick Offset: layer.quickOffset and keys.quickOffset commands (#1012)

After Effects 25.4's Quick Offset staggers selected layers, or each
layer's selected keyframes, in the order they were selected: the first
stays, the last moves by the drag and the ones between by equal steps,
landing on subframes.

layer.quickOffset {delta, layers?} moves each layer's start, in and out
point by total * i / (n - 1), worked out once in ticks with integer
arithmetic, so 1 frame over 4 layers gives 0, 1/3, 2/3 and 1 frame
exactly. keys.quickOffset {delta} does the same per layer for the
selected keys, the layers in the order their first key was selected;
every selected key of a layer, across its properties, moves by that
layer's comp-time offset through its start and stretch (reversed too),
keeping values, interpolation, easing and labels. Locked layers are
skipped and at least two layers are needed. With merge and fromStart,
delta is the whole drag and each step starts again from before it, so
the drag is one undo step and a key passed over survives. A delta or a
destination out of the range of times is an error, not a clamped move.

Docs: agents.md; architecture.md notes Quick Offset as the exception to
times snapping to frames.

docs/architecture.md

@@ -72,7 +72,9 @@ All time is an integer `Tick`, 254 016 000 000 per second, the least common mult
 broadcast frame rate (×1001) and audio sample rate, so frame and sample positions are exact.
 Keyframe times are in **layer time**; `layer_time = (comp_time - start_time) / stretch`, and a
 Time Remap property overrides the source time. Commands snap layer times, keyframe times and
-comp durations to whole frames, as After Effects does. Timecode is only a display.
+comp durations to whole frames, as After Effects does, except Quick Offset (`layer.quickOffset`,
+`keys.quickOffset`), which staggers layers and keys onto subframes, as After Effects 25.4 does.
+Timecode is only a display.
 
 ## 3. Document model