94Implement an analytics SDK with event batching and sendBeacon fallback

Implement createAnalytics({ endpoint, batchSize = 10, flushInterval = 5000, maxQueue = 500, transport, target = window, doc = document }) returning { track(name, props), flush(), destroy(), queueSize }.

• track adds { id, name, props, ts } to a queue (id from crypto.randomUUID(), ts from Date.now()).
• The queue is sent as one request when it reaches batchSize, and at least every flushInterval ms while it is not empty.
• Normal sends use transport.fetch(endpoint, { method: "POST", body, keepalive: true, headers }) with a JSON body { events: [...] }. If it fails (network error or non-2xx), put the events back at the front of the queue so they are sent next time.
• When the page is being hidden or unloaded (visibilitychange with doc.visibilityState === "hidden", or pagehide on target), flush with transport.sendBeacon(endpoint, blob). If sendBeacon returns false (or is missing), fall back to the keepalive fetch.
• The queue never holds more than maxQueue events; drop the oldest when it overflows.
• destroy() stops the timer and removes the listeners.

transport defaults to { fetch: window.fetch.bind(window), sendBeacon: navigator.sendBeacon?.bind(navigator) }; tests pass fakes.