feat: F4.6f2 the text editor on the document's Y.Text, files both ways while the simulation runs

Oracle TextEditorWindowRun uses Java's text editor window and its file
dialog as a person does. The open file's text is its Y.Text in the
document (plan 7.3): typed is in the file at once, for everyone and for
the running engine. Core openText/saveTextAs without Java's mistakes
(lost saves after save-as onto a file or a removed file, files named
'ordner/d.txt' or ''); doc files.ts (changeFiles, fileText, nodeFiles,
readFilesInto); the room takes a person's files into the engine at once
and writes the engine's files back at the tick they changed; the editor's
window, Java's dialog, the text area following others' changes with the
caret in place, its own undo.

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

packages/doc/src/projectDoc.ts

@@ -224,7 +224,7 @@ export function inOrder(map: Y.Map<Y.Map<unknown>>): [string, Y.Map<unknown>][]
  * system refuses (its folder missing before the janitor ran, or a name two
  * people gave at once in different case) is left out, not the project.
  */
-function fileSystemOf(files: Y.Map<Y.Map<unknown>>): VirtualFileSystem {
+export function fileSystemOf(files: Y.Map<Y.Map<unknown>>): VirtualFileSystem {
   const fileSystem = new VirtualFileSystem();
   for (const [path, entry] of inOrder(files)) {
     const kind = entry.get('kind') as VirtualFileContent['kind'];