perf: Defer OTel CDN loading to after first paint

Moved OpenTelemetry SDK loading from blocking script execution to
requestIdleCallback (with setTimeout fallback). This ensures:
- First paint is not blocked by CDN latency
- OTel loads during browser idle time after initial render
- Graceful degradation if CDN is slow or unavailable

The shim ensures all tracing calls work immediately, with real
implementation hydrated asynchronously after first paint.

Author: MCGPPeters