feat(editor): a computer's files to and from the person's own files (F8.1)

Beside the dashboard only: Java's import window gets "Aus meinen Dateien"
(D4) — the dashboard is asked over the bridge for a file of at most Java's
150 000 bytes and hands its bytes over — and a file's menu gets "In meine
Dateien speichern" (D5), its bytes handed to the dashboard to save.

- Bridge: chooseFile/chosenFile and saveFile/savedFile, each question with
  its number; an answer from elsewhere, of another version, for no question
  or of the other kind is no answer.
- Whoever reads or finds the project frozen may save, not bring in.
- doc fileBytes: a text as UTF-8, a picture as it is, and "missing" while
  a blob's bytes have not come (read so, the file would be empty).

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

packages/editor/test/bridge.test.ts

@@ -2,15 +2,15 @@
 // and the editor says when it listens.
 
 import { describe, expect, it } from 'vitest';
-import { BRIDGE_VERSION, startBridge } from '../src/bridge/dashboardBridge.js';
+import { BRIDGE_VERSION, dashboardFiles, startBridge } from '../src/bridge/dashboardBridge.js';
 
 function fakeWindow() {
-  const posted: [unknown, string][] = [];
+  const posted: [unknown, string, Transferable[]?][] = [];
   const listeners = new Set<(event: { origin: string; data: unknown }) => void>();
   return {
     posted,
     window: {
-      parent: { postMessage: (message: unknown, origin: string) => posted.push([message, origin]) },
+      parent: { postMessage: (message: unknown, origin: string, transfer?: Transferable[]) => posted.push(transfer === undefined ? [message, origin] : [message, origin, transfer]) },
       addEventListener: (_: 'message', listener: (event: { origin: string; data: unknown }) => void) => listeners.add(listener),
       removeEventListener: (_: 'message', listener: (event: { origin: string; data: unknown }) => void) => listeners.delete(listener),
     },
@@ -51,3 +51,59 @@ describe('the bridge to the dashboard', () => {
     expect(fake.listening()).toBe(0);
   });
 });
+
+describe("the person's own files over the bridge (F8)", () => {
+  const DASHBOARD = 'https://dashboard.example';
+
+  it('asks for a file with its limit and takes the answer to that question only', async () => {
+    const fake = fakeWindow();
+    const files = dashboardFiles(fake.window, DASHBOARD);
+    const chosen = files.chooseFile(150_000);
+    expect(fake.posted).toEqual([[{ v: BRIDGE_VERSION, type: 'chooseFile', request: 1, maxBytes: 150_000 }, DASHBOARD, []]]);
+    const data = new Uint8Array([1, 2, 3]).buffer;
+    // From elsewhere, of another version, for another question, of the other kind: no answer — the question still waits.
+    fake.send('https://evil.example', { v: 1, type: 'chosenFile', request: 1, name: 'x.png', data });
+    fake.send(DASHBOARD, { v: 2, type: 'chosenFile', request: 1, name: 'x.png', data });
+    fake.send(DASHBOARD, { v: 1, type: 'chosenFile', request: 9, name: 'x.png', data });
+    fake.send(DASHBOARD, { v: 1, type: 'savedFile', request: 1, result: 'saved', name: 'x.png' });
+    fake.send(DASHBOARD, { v: 1, type: 'chosenFile', request: 1, name: 'Bild.png', data });
+    expect(await chosen).toEqual({ name: 'Bild.png', bytes: new Uint8Array([1, 2, 3]) });
+  });
+
+  it('hears a cancelled choice as none', async () => {
+    const fake = fakeWindow();
+    const files = dashboardFiles(fake.window, DASHBOARD);
+    const chosen = files.chooseFile(10);
+    fake.send(DASHBOARD, { v: 1, type: 'chosenFile', request: 1, cancelled: true });
+    expect(await chosen).toBeNull();
+  });
+
+  it('hands a copy of the bytes over to save, and keeps its own', async () => {
+    const fake = fakeWindow();
+    const files = dashboardFiles(fake.window, DASHBOARD);
+    const bytes = new Uint8Array([9, 8, 7, 6]).subarray(1, 3);
+    const saved = files.saveFile('Grüße.txt', bytes);
+    const [message, origin, transfer] = fake.posted[0]!;
+    expect(origin).toBe(DASHBOARD);
+    expect(message).toMatchObject({ v: BRIDGE_VERSION, type: 'saveFile', request: 1, name: 'Grüße.txt' });
+    const data = (message as { data: ArrayBuffer }).data;
+    expect([...new Uint8Array(data)]).toEqual([8, 7]);
+    expect(transfer).toEqual([data]);
+    expect(data).not.toBe(bytes.buffer);
+    fake.send(DASHBOARD, { v: 1, type: 'savedFile', request: 1, result: 'saved', name: 'Grüße (2).txt' });
+    expect(await saved).toEqual({ result: 'saved', name: 'Grüße (2).txt' });
+  });
+
+  it('counts its questions, and an unknown result is a failure', async () => {
+    const fake = fakeWindow();
+    const files = dashboardFiles(fake.window, DASHBOARD);
+    const first = files.saveFile('a.txt', new Uint8Array());
+    const second = files.saveFile('b.txt', new Uint8Array());
+    fake.send(DASHBOARD, { v: 1, type: 'savedFile', request: 2, result: 'cancelled' });
+    fake.send(DASHBOARD, { v: 1, type: 'savedFile', request: 1, result: 'weird' });
+    expect(await second).toEqual({ result: 'cancelled', name: 'b.txt' });
+    expect(await first).toEqual({ result: 'failed', name: 'a.txt' });
+    files.stop();
+    expect(fake.listening()).toBe(0);
+  });
+});