M0.13: viewer snapping, rulers, channels, snapshots, ROI

Composition viewer interaction parity with After Effects:
- Snapping (View > Snapping, the Tools bar checkbox, Cmd/Ctrl inverts while
  dragging): layer edges, centres and anchor points, mask and shape-path
  vertices, comp edges/centre, guides (Snap to Guides) and grid (Snap to
  Grid), with highlighted target box / line / cross feedback. Pure maths in
  engine::viewer (nearest target within tolerance, priority ties).
- Rulers (Cmd+R) with live pointer markers and a draggable zero point;
  guides dragged out of the rulers, moved in the viewer, dragged off to
  remove (view.moveGuide / view.removeGuide), lock respected; proportional
  grid.
- Show Channel (RGB/Red/Green/Blue/Alpha/RGB Straight, Colorize), exposure
  (+/- stops in linear light, reset), Take Snapshot (Shift+F5) with a stored
  render and Show Snapshot (F5 held / button held), Fast Previews (Off,
  Adaptive Resolution, Draft, Fast Draft, Wireframe), Resolution > Custom.
- Region of interest: drawn in the viewer; RenderOpts.roi renders only that
  rectangle (crop of the full render, tested), Crop Comp to ROI uses it.
- Bottom bar in AE 2026 order ("136.1 %", "(Full)").
- Shape Pen (SHP-1): Pen on a shape layer / nothing selected draws Shape n
  groups (Path 1, Stroke 1, Fill 1) via shape.newPath; the G slot cycles
  Add / Delete / Convert Vertex and Mask Feather; mask.* vertex commands
  work on shape paths; mask.insertVertex / mask.convertVertex; marquee
  vertex selection, Alt-drag/click converts, double-click free transform
  box (path.freeTransform on masks and shape paths).
- Motion paths (ANM-3): keyframe dots and spatial tangent handles, dragging
  keys (keys.set) and tangents (keys.setSpatialTangents), parented layers.

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

docs/control-protocol.md

@@ -81,7 +81,21 @@ Element ids are stable, for example `tools.Selection`, `panel.Timeline`, `panel.
 `header.workspace.Animation`, `project.item.<id>`, `viewer.comp`, `viewer.handle.<layer>.<i>`,
 `timeline.layer.<id>.bar`, `timeline.layer.<id>.twirl`, `timeline.prop.<uid>.stopwatch`,
 `timeline.key.<uid>.<frame>`, `timeline.cti`, `effectControls.prop.<uid>.value` and
-`effects.item.<name>`. Use `ui.elements` to see what is on screen.
+`effects.item.<name>`. The Composition viewer adds `viewer.magnification`, `viewer.resolution`,
+`viewer.roi`, `viewer.grid`, `viewer.channel`, `viewer.exposure`, `viewer.snapshot`,
+`viewer.showSnapshot`, `viewer.fastPreviews` (their popup entries are `viewer.<menu>Item.<n>`),
+`viewer.ruler.top|left|origin`, `viewer.mask.<uid>.vertex.<i>`, `viewer.shapePath.<uid>.vertex.<i>`,
+`viewer.motionPath.<layer>.<key>[.in|.out]`, `viewer.freeTransform.handle.<i>` and
+`viewer.regionOfInterest`; the Graph Editor adds `timeline.graph.transformBox[.<i>]`,
+`timeline.graph.snap` and `timeline.graph.reference`. Use `ui.elements` to see what is on screen.
+
+Viewer state that agents drive headless too: `view.snapping`, `view.channel {channel, colorized?}`,
+`view.exposure {stops | delta}`, `view.resetExposure`, `view.takeSnapshot`, `view.showSnapshot`,
+`view.fastPreviewMode {mode}`, `view.setRegionOfInterest {rect}`, `view.addGuide`,
+`view.moveGuide`, `view.removeGuide`; editing: `shape.newPath` (Pen on shape layers),
+`mask.insertVertex`, `mask.convertVertex`, `mask.deleteVertices`, `path.freeTransform` (masks and
+shape paths by uid), `keys.setSpatialTangents` (motion-path handles) and `keys.transform` (Graph
+Editor transform box, timeline Alt-drag scaling).
 
 The MCP server's bridge mode (`effectcraft-cli mcp --bridge 9877`) is a thin client of this protocol;
 see [agents.md](agents.md).