feat: fractional indexing (#7359)

* Introducing fractional indices as part of `element.index`

* Ensuring invalid fractional indices are always synchronized with the array order

* Simplifying reconciliation based on the fractional indices

* Moving reconciliation inside the `@excalidraw/excalidraw` package

---------

Co-authored-by: Marcel Mraz <marcel@excalidraw.com>
Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/excalidraw/scene/export.ts

@@ -38,6 +38,7 @@ import { Mutable } from "../utility-types";
 import { newElementWith } from "../element/mutateElement";
 import { isFrameElement, isFrameLikeElement } from "../element/typeChecks";
 import { RenderableElementsMap } from "./types";
+import { syncInvalidIndices } from "../fractionalIndex";
 import { renderStaticScene } from "../renderer/staticScene";
 
 const SVG_EXPORT_TAG = `<!-- svg-source:excalidraw -->`;
@@ -224,7 +225,7 @@ export const exportToCanvas = async (
       arrayToMap(elementsForRender),
     ),
     allElementsMap: toBrandedType<NonDeletedSceneElementsMap>(
-      arrayToMap(elements),
+      arrayToMap(syncInvalidIndices(elements)),
     ),
     visibleElements: elementsForRender,
     scale,