Timeline: trim all selected layers together

Dragging the in or out edge of a selected layer's bar now trims every
selected layer by the same delta from its own in/out point, as in After
Effects. layer.timing gains relative inDelta/outDelta params (seconds,
frame-snapped), clamped to the comp start/end and to one frame of layer;
the drag stays one undo step via the merge key.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/agents.md

@@ -144,7 +144,7 @@ carries outside-range warnings; the existing objects from `keys.set` and `prop.g
 also report both bases and warnings. `prop.get.time` is comp seconds. Separated Position
 `prop.set` retains per-axis replies; for these commands, `path` or `prop` identifies a property, not a group.
 
-Set offsets with `layer.timing {layer, start: 10.5}`; `in`, `out` and `delta` also use comp seconds.
+Set offsets with `layer.timing {layer, start: 10.5}`; `in`, `out` and `delta` also use comp seconds; `inDelta` / `outDelta` trim each layer from its own in/out point.
 For an unstretched layer starting at 10.5 s in a 30 fps comp,
 `prop.addKey {layer: "A", path: "transform/opacity", time: 15.3,
 timeBase: "comp", value: 0}` writes a key at comp 15.3 s (layer 4.8 s), matching

docs/architecture.md

@@ -388,6 +388,9 @@ their on-screen scale, as text is vector in After Effects; on shape layers and v
 switch is Continuously Rasterize and does the same. Quality: Draft samples
 nearest-neighbour, Wireframe draws the layer bounds. Slip edit (`layer.slip`, Alt+PageUp/Down,
 dragging the source bar in the timeline) moves the source under fixed in/out points.
+Dragging a layer bar's in or out edge with several layers selected trims every selected layer by
+the same amount from its own in/out point (`layer.timing {inDelta | outDelta}`), clamped to the
+comp and to one frame of layer, as one undo step.
 
 **Nested comps** show nothing before they start or after they end (`Comp::covers`), even when
 the precomp layer is trimmed or remapped past them. A comp with **Preserve frame rate when nested