feat: show update download telemetry

Calculate download throughput and remaining time from verified installer bytes and display downloaded size, total size, speed, and ETA beneath the progress track while retaining the last values after cancellation.

tests/updater-version.test.js

@@ -103,6 +103,7 @@ test('buffered installer downloads yield between progress updates so the rendere
   let preparationFinished = false;
   let rendererObservedProgressBeforeFinish = false;
   let rendererObservationScheduled = false;
+  let nowMs = 0;
 
   try {
     await prepareUpdate(value => {
@@ -121,6 +122,7 @@ test('buffered installer downloads yield between progress updates so the rendere
         latestYmlUrl: 'https://update.invalid/latest.yml'
       },
       tempDir,
+      now: () => nowMs,
       fetchImpl: async url => url.endsWith('latest.yml')
         ? {
             ok: true,
@@ -132,9 +134,12 @@ test('buffered installer downloads yield between progress updates so the rendere
             status: 200,
             body: {
               getReader: () => ({
-                read: async () => readerIndex < chunks.length
-                  ? { done: false, value: chunks[readerIndex++] }
-                  : { done: true }
+                read: async () => {
+                  nowMs += 1000;
+                  return readerIndex < chunks.length
+                    ? { done: false, value: chunks[readerIndex++] }
+                    : { done: true };
+                }
               })
             }
           }
@@ -147,6 +152,10 @@ test('buffered installer downloads yield between progress updates so the rendere
       progress.filter(value => value.stage === 'downloading').map(value => value.percent),
       [25, 50, 75, 100]
     );
+    assert.deepEqual(
+      progress.filter(value => value.stage === 'downloading').map(value => [value.bytesPerSecond, value.etaSeconds]),
+      [[32768, 3], [32768, 2], [32768, 1], [32768, 0]]
+    );
   } finally {
     fs.rmSync(tempDir, { recursive: true, force: true });
   }