feat(editor): add transient app state foundation

editor/index.ts

@@ -0,0 +1 @@
+export type { AppState, EditorState, SelectionState, ViewportState } from "./state";

editor/state.ts

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+import type { ImageDocument } from "@core/document";
+import type { Angle, Size, Vec2D } from "@core/geometry";
+import type { ArtboardId, LayerId } from "@core/id";
+
+export type ViewportState = {
+  center: Vec2D;
+  zoom: number;
+  rotation: Angle;
+  size: Size;
+};
+
+export type SelectionState = {
+  artboardId?: ArtboardId;
+  layerIds: LayerId[];
+};
+
+export type EditorState = {
+  viewport: ViewportState;
+  selection: SelectionState;
+};
+
+export type AppState = {
+  document: ImageDocument;
+  editor: EditorState;
+};