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 <noreply@anthropic.com>

docs/architecture.md

@@ -172,7 +172,10 @@ options are never shown. When the budget (Settings ▸ Memory & CPU) is full the
 shown frames go first, and frames of an older revision are dropped as soon as the project
 changes. The timeline's green bar counts the frames of exactly what the viewer shows.
 Resolution Auto renders the pixels the magnification needs, as in After Effects (Full above
-50 %, Half down to 33.3 %, Third down to 25 %, then Quarter). Below 100 % the viewer averages
+50 %, Half down to 33.3 %, Third down to 25 %, then Quarter). The Resolution is the comp's
+(`Comp::resolution`, saved with the project): setting it is no undo step (undo, redo and history
+jumps carry the current values over) and, like the Shy switches, it is outside
+`Comp::same_pixels`, so cached frames stay valid. Below 100 % the viewer averages
 instead of skipping pixels (Viewer Zoom Quality More Accurate): GPU frames carry a mip chain
 sampled trilinearly, CPU frames go up averaged down by the whole factor that leaves about one
 texel per screen pixel.