perf(ui): enforce 750 ms live update cadence

Use one 750 ms interval for running download snapshots, progress updates, the bandwidth chart, and the header speed sparkline across every queue size. Remove the extra active-download renderer delay and add timing regressions for small, medium, and large queues.

src/main/download-manager.ts

@@ -163,8 +163,9 @@ const ARCHIVE_SETTLE_MAX_WAIT_MS = 5000;
 
 const MAX_SAME_DIRECT_URL_ATTEMPTS = 3;
 
-const MAX_HTTP416_FRESH_RESTARTS = 2;
-const HTTP416_FRESH_RESTART_DELAY_MS = 8000;
+const MAX_HTTP416_FRESH_RESTARTS = 2;
+const HTTP416_FRESH_RESTART_DELAY_MS = 8000;
+const DOWNLOAD_LIVE_UPDATE_INTERVAL_MS = 750;
 
 function getHttp416FreshRestartDelayMs(): number {
   const fromEnv = Number(process.env.RD_HTTP416_FRESH_RESTART_DELAY_MS ?? NaN);
@@ -6561,14 +6562,7 @@ export class DownloadManager extends EventEmitter {
     if (this.stateEmitTimer) {
       return;
     }
-    const itemCount = this.itemCount;
-    const emitDelay = this.session.running
-      ? itemCount >= 1500
-        ? 700
-        : itemCount >= 250
-          ? 500
-          : 150
-      : 200;
+    const emitDelay = this.session.running ? DOWNLOAD_LIVE_UPDATE_INTERVAL_MS : 200;
     this.stateEmitTimer = setTimeout(() => {
       this.stateEmitTimer = null;
       this.lastStateEmitAt = nowMs();
@@ -10579,12 +10573,7 @@ export class DownloadManager extends EventEmitter {
         let windowStarted = nowMs();
         let lastPackageLogAt = 0;
         let lastLoggedPercent = -1;
-        const itemCount = this.itemCount;
-        const uiUpdateIntervalMs = itemCount >= 1500
-          ? 700
-          : itemCount >= 250
-            ? 500
-            : 120;
+        const uiUpdateIntervalMs = DOWNLOAD_LIVE_UPDATE_INTERVAL_MS;
         let lastUiEmitAt = 0;
         const stallTimeoutMs = getDownloadStallTimeoutMs();
         const drainTimeoutMs = Math.max(30000, Math.min(300000, stallTimeoutMs > 0 ? stallTimeoutMs * 12 : 120000));