feat(web): surface QEMU logs in-page and enable guest-error diagnostics

Add a log panel that captures QEMU stdout/stderr, route serial to stdio, and
pass -d guest_errors,unimp so device warnings (e.g. the m25p80 Read-ID gap that
stalls CrossPoint boot) are visible in the browser. Read the framebuffer via
HEAPU8 to match the validated node path.

web/app.js

@@ -76,7 +76,7 @@ function drawFrame(module, canvas) {
     canvas.height = height;
   }
 
-  const source = new Uint8Array(module.wasmMemory.buffer);
+  const source = module.HEAPU8;
   const image = new ImageData(width, height);
   for (let y = 0; y < height; y++) {
     for (let x = 0; x < width; x++) {
@@ -153,6 +153,24 @@ function enableControls(module) {
   }
 }
 
+let logEl = null;
+const logBuffer = [];
+const LOG_MAX_LINES = 1000;
+
+function logLine(message) {
+  logBuffer.push(message);
+  if (logBuffer.length > LOG_MAX_LINES) {
+    logBuffer.splice(0, logBuffer.length - LOG_MAX_LINES);
+  }
+  if (logEl) {
+    const atBottom = logEl.scrollTop + logEl.clientHeight >= logEl.scrollHeight - 4;
+    logEl.textContent = logBuffer.join('\n');
+    if (atBottom) {
+      logEl.scrollTop = logEl.scrollHeight;
+    }
+  }
+}
+
 async function boot(file, variant, setStatus) {
   if (!crossOriginIsolated) {
     throw new Error('This emulator requires COOP/COEP headers. Start it with `make web`.');
@@ -175,14 +193,15 @@ async function boot(file, variant, setStatus) {
       '-machine', `xteink,variant=${variant}`,
       '-L', '/bios',
       '-display', 'none',
-      '-serial', 'null',
+      '-serial', 'stdio',
       '-nic', 'none',
+      '-d', 'guest_errors,unimp',
       '-drive', 'file=/flash.bin,if=mtd,format=raw',
     ],
     locateFile: path => new URL(path, artifactRoot).href,
     mainScriptUrlOrBlob: qemuUrl.href,
-    print: message => console.log(message),
-    printErr: message => console.error(message),
+    print: message => { console.log(message); logLine(message); },
+    printErr: message => { console.error(message); logLine(message); },
   };
   options.preRun = [() => {
     options.FS.mkdir('/bios');
@@ -201,6 +220,11 @@ function initialize() {
   const bootButton = document.querySelector('#boot');
   const status = document.querySelector('#status');
   const canvas = document.querySelector('#screen');
+  logEl = document.querySelector('#log');
+  document.querySelector('#log-clear').addEventListener('click', () => {
+    logBuffer.length = 0;
+    logEl.textContent = '';
+  });
 
   const setStatus = (message, error = false) => {
     status.textContent = message;

web/index.html

@@ -48,6 +48,14 @@
         <button type="button" class="power" data-button="power" disabled>Power</button>
       </div>
     </section>
+
+    <section class="log-panel" aria-label="Emulator logs">
+      <div class="log-header">
+        <span>QEMU log</span>
+        <button id="log-clear" type="button">Clear</button>
+      </div>
+      <pre id="log" class="log" role="log" aria-live="polite"></pre>
+    </section>
   </main>
 
   <script type="module" src="app.js"></script>

web/styles.css

@@ -69,3 +69,8 @@ canvas { display: block; width: 100%; height: auto; background: #fff; image-rend
   .device { grid-template-columns: 1fr; }
   .controls { max-width: 24rem; width: 100%; margin: 0 auto; }
 }
+
+.log-panel { margin-top: 2rem; }
+.log-header { display: flex; justify-content: space-between; align-items: center; margin-bottom: .5rem; font-size: .85rem; color: #b9b3a4; }
+.log-header button { min-height: auto; padding: .25rem .75rem; font-size: .8rem; }
+.log { margin: 0; max-height: 20rem; overflow: auto; padding: .75rem 1rem; border-radius: .6rem; background: #1c1d1a; color: #d6d0c2; font: .78rem/1.4 ui-monospace, monospace; white-space: pre-wrap; word-break: break-word; }