feat(sdk): implement inline stub loader

Phase 1: Inline Stub (2h)
- Event queue implementation
- Rush stub with init/track/trackPageview methods
- Auto-init from data-site attribute
- Dynamic script injection (async)
- onload/onerror handlers
- Expose queue via window.Rush._queue
- Debug mode logging
- Configurable SDK URL via data-sdk-url

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: rmarsigli

sdk/stub.js

@@ -0,0 +1,60 @@
+(function(window, document) {
+  'use strict';
+
+  // 1. Configuration from script tag
+  var script = document.currentScript || document.querySelector('script[data-site]');
+  var siteId = script && script.dataset.site;
+  var endpoint = script && script.dataset.endpoint || 'https://analytics.example.com/ingest';
+  var sdkUrl = script && script.dataset.sdkUrl || 'https://cdn.example.com/rush.min.js';
+  var debug = script && script.dataset.debug === 'true';
+
+  // 2. Event queue (stores calls before SDK loads)
+  var queue = [];
+
+  // 3. Stub Rush object (queues all calls)
+  window.Rush = {
+    init: function(config) {
+      queue.push(['init', config]);
+    },
+    track: function(eventName, data) {
+      queue.push(['track', eventName, data]);
+    },
+    trackPageview: function() {
+      queue.push(['trackPageview']);
+    }
+  };
+
+  // 4. Auto-init if data-site provided
+  if (siteId) {
+    window.Rush.init({
+      siteId: siteId,
+      endpoint: endpoint,
+      debug: debug,
+      autoTrack: script.dataset.autoTrack !== 'false'
+    });
+  }
+
+  // 5. Inject external SDK script
+  var sdkScript = document.createElement('script');
+  sdkScript.src = sdkUrl;
+  sdkScript.async = true;
+  sdkScript.onload = function() {
+    // SDK loaded, it will process the queue
+    if (debug) console.log('[Rush] SDK loaded, processing queue:', queue.length, 'events');
+  };
+  sdkScript.onerror = function() {
+    if (debug) console.error('[Rush] Failed to load SDK');
+  };
+
+  // Inject before first script or in head
+  var firstScript = document.getElementsByTagName('script')[0];
+  if (firstScript) {
+    firstScript.parentNode.insertBefore(sdkScript, firstScript);
+  } else {
+    document.head.appendChild(sdkScript);
+  }
+
+  // 6. Expose queue for SDK to process
+  window.Rush._queue = queue;
+
+})(window, document);