Viewer: each composition keeps its Resolution in the project, as in After Effects (#417)

What: the Composition panel's Resolution (Auto, Full, Half, Third, Quarter, Custom) is now a
property of the composition (`Comp::resolution`, moved with its `Resolution` type from the UI
into the project crate) instead of one global viewer setting. Switching to another comp shows
that comp's resolution, switching back shows the first one's again, and saving and reopening the
project keeps them. `view.res.full|half|third|quarter|custom` and the new engine command
`view.res.auto` set it (headless too, with an optional `comp`); `ui.set {viewer: {res}}` sets
the active comp's and `ui.inspect` still reports `ui.viewer.res`; `comp.info` reports
`resolution` and `resolutionFactor`; View ▸ Resolution's check marks come from the engine.

Why: After Effects saves each composition's Resolution with the project (CompItem.resolutionFactor
is a per-comp attribute). As there, changing it marks the project modified but adds no undo step:
undo, redo and History jumps carry the current values over to the restored state. It is not part
of the comp's pixels (`Comp::same_pixels` ignores it and `Comp::pixel_form` normalises it), so
changing it keeps the comp's RAM and disk cache keys. Auto is not written (serde default and
skip_serializing_if), so projects saved before open with Auto and older versions read new files.
The magnification is not saved per comp: Adobe's documentation doesn't say it is.

Tests: project `comp_resolution_is_saved_and_old_projects_load_with_auto`,
`resolution_factors_and_scales`; engine
`each_comp_keeps_its_resolution_outside_the_undo_history_and_in_the_file` (switching comps,
no undo step, undo/redo keep it, save/open round trip, menu check marks, Custom dialog);
ui-egui `each_comp_keeps_its_viewer_resolution` (the viewer popup on comp A, comp B through
`ui.set`, back to A, unchanged frame cache identity, save/open). They don't build before the
change (no `Comp::resolution`, no `view.res.auto` engine command).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/control-protocol.md

@@ -114,7 +114,10 @@ Viewer state that agents drive headless too: `view.snapping`, `view.snappingOpti
 `header.snappingOptions`: After Effects' Snap Edges Extended and Snap to Features in Collapsed
 Compositions and Text Layers), `view.channel {channel, colorized?}`,
 `view.exposure {stops | delta}`, `view.resetExposure`, `view.takeSnapshot`, `view.showSnapshot`,
-`view.fastPreviewMode {mode}`, `view.setRegionOfInterest {rect}`, `view.addGuide`,
+`view.fastPreviewMode {mode}`, `view.res.full|half|third|quarter|auto {comp?}` and
+`view.res.custom {factor, comp?}` (the comp's Resolution, saved with the project but no undo
+step; `ui.set {viewer: {res}}` sets the active comp's, and `ui.inspect` reports it as
+`ui.viewer.res`), `view.setRegionOfInterest {rect}`, `view.addGuide`,
 `view.moveGuide`, `view.removeGuide`; editing: `shape.newPath` (Pen on shape layers),
 `shape.newShape` (the shape tools: a new group in the selected shape layer, else a new shape layer),
 `shape.toolOptions` (the Tools bar's Tool Creates Shape / Mask and Fill and Stroke Options),