feat(diag): --debug ASIO routing diagnostics ([asio-diag])

Testers report song/game audio on the default WASAPI device while the
guitar rides the selected ASIO output. The renderer-side reroute chain
([feedpak-route]/[renderer-bus]) has change-gated logs, but the native
side of the story (what the engine actually opened, whether the backing
transport/renderer bus is live) was invisible in tester logs.

- audio-bridge: debug-gated 2s engine snapshot, change-gated + 30s
  heartbeat: device types/names, duplex, backing, renderer-bus health
- debug:isEnabled IPC + preload audio.debugEnabled() so the static
  bundle can gate its verbose lines on --debug

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/audio-bridge.ts

@@ -325,6 +325,56 @@ export function initAudioBridge(): void {
 
     ipcMain.handle('audio:isAvailable', () => audio !== null);
 
+    // Renderer-side diagnostics gate: the static bundle's routing watcher /
+    // renderer-bus feeder emit verbose [asio-diag] lines only when the app
+    // runs with --debug / --verbose / SLOPSMITH_DEBUG (issue: ASIO output —
+    // song audio heard on the default WASAPI device while guitar rides ASIO).
+    ipcMain.handle('debug:isEnabled', () => isDebugEnabled());
+
+    // [asio-diag] main-process routing snapshot. Polls the engine every 2s in
+    // debug mode and logs a one-line snapshot whenever the routing-relevant
+    // state changes (plus a 30s heartbeat so a log without transitions still
+    // proves the watcher was alive). Captures the native side of the story the
+    // renderer diagnostics can't see: which device types/names the engine
+    // actually opened, whether the backing transport is playing, and whether
+    // the renderer bus is enabled and moving frames.
+    if (audio && isDebugEnabled()) {
+        let lastSnapshot = '';
+        let lastLogged = 0;
+        setInterval(() => {
+            try {
+                if (!audio) return;
+                const running = !!audio.isAudioRunning?.();
+                const dev = running ? audio.getCurrentDevice?.() : null;
+                const bus = audio.getRendererBusMetrics?.() ?? null;
+                const snapshot = JSON.stringify({
+                    running,
+                    inputType: dev?.inputType ?? '',
+                    outputType: dev?.outputType ?? '',
+                    input: dev?.input ?? '',
+                    output: dev?.output ?? '',
+                    duplex: dev?.duplex ?? null,
+                    backingPlaying: !!audio.isBackingPlaying?.(),
+                    streamOutputActive: !!audio.isStreamOutputActive?.(),
+                    busEnabled: bus?.enabled ?? null,
+                    // pushed/consumed prove frames are flowing; deltas matter,
+                    // absolute counts churn — bucket to "moving or not".
+                    busFlowing: !!bus && bus.pushedFrames > 0 && bus.consumedFrames > 0,
+                    busUnderflows: bus?.underflowCount ?? null,
+                    busOverflows: bus?.overflowCount ?? null,
+                });
+                const now = Date.now();
+                if (snapshot !== lastSnapshot || now - lastLogged > 30000) {
+                    lastSnapshot = snapshot;
+                    lastLogged = now;
+                    console.log(`[asio-diag] engine: ${snapshot}`);
+                }
+            } catch (e: any) {
+                console.warn(`[asio-diag] snapshot failed: ${e.message}`);
+            }
+        }, 2000);
+    }
+
     // ── Device Management ──────────────────────────────────────────────────
 
     ipcMain.handle('audio:getDeviceTypes', () => {

src/main/preload.ts

@@ -212,6 +212,10 @@ const feedBackDesktopApi = {
             ? ipcRenderer.invoke('audio:probeDeviceOptions', inputTypeOrType, inputName, outputTypeOrOutputName)
             : ipcRenderer.invoke('audio:probeDeviceOptions', inputTypeOrType, inputName, outputTypeOrOutputName, outputName),
         getCurrentDevice: () => ipcRenderer.invoke('audio:getCurrentDevice'),
+        // True when the app runs with --debug / --verbose / SLOPSMITH_DEBUG.
+        // The static bundle gates its verbose [asio-diag] routing lines on
+        // this so normal runs don't spam the console.
+        debugEnabled: (): Promise<boolean> => ipcRenderer.invoke('debug:isEnabled'),
         setDeviceType: (typeName: string) => ipcRenderer.invoke('audio:setDeviceType', typeName),
         setOutputDeviceType: (typeName: string) => ipcRenderer.invoke('audio:setOutputDeviceType', typeName),
         setDevice: ((