Add url-sync helper + wire into viewer

src/lib/TraceViewer.svelte

@@ -2,6 +2,7 @@
   import { tick, onMount, onDestroy } from 'svelte';
   import { parseJsonl, toRawUrl } from './parse.js';
   import { renderMarkdown } from './markdown.js';
+  import { getParam, setParam } from './url-sync.js';
 
   const spinnerFrames = ['⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏'];
   let spinnerFrame = $state(0);
@@ -73,6 +74,7 @@
       if (parsed.length === 0) {
         error = 'No messages parsed from this file.';
       } else {
+        setParam('url', url);
         await tick();
         playback();
       }
@@ -236,6 +238,12 @@
     spinnerInterval = setInterval(() => {
       spinnerFrame = (spinnerFrame + 1) % spinnerFrames.length;
     }, 90);
+
+    const shared = getParam('url');
+    if (shared) {
+      url = shared;
+      load();
+    }
   });
 
   onDestroy(() => {

src/lib/url-sync.js

@@ -0,0 +1,46 @@
+// Sync shareable URL query params between this iframe and the HF Spaces
+// parent frame (huggingface.co/spaces/<owner>/<name>?...).
+//
+// Pattern borrowed from lerobot-dataset-visualizer:
+// - On mount, read the iframe's own query string (HF forwards the parent's
+//   params into the iframe's URL on first load).
+// - On state change, update our own URL via history.replaceState AND
+//   postMessage({ queryString }) to the parent so the shareable link on
+//   huggingface.co reflects our state.
+
+const HF_PARENT_ORIGIN = 'https://huggingface.co';
+
+export function getParam(key) {
+  try {
+    return new URLSearchParams(window.location.search).get(key);
+  } catch {
+    return null;
+  }
+}
+
+export function setParam(key, value) {
+  try {
+    const params = new URLSearchParams(window.location.search);
+    if (value == null || value === '') params.delete(key);
+    else params.set(key, value);
+    const qs = params.toString();
+    const newUrl = `${window.location.pathname}${qs ? '?' + qs : ''}`;
+    window.history.replaceState(null, '', newUrl);
+    postToParent(params);
+  } catch {
+    // Ignore — standalone deploy or cross-origin blocked.
+  }
+}
+
+function postToParent(params) {
+  try {
+    if (window.parent && window.parent !== window) {
+      window.parent.postMessage(
+        { queryString: params.toString() },
+        HF_PARENT_ORIGIN
+      );
+    }
+  } catch {
+    // Ignore.
+  }
+}