feat(app): OpenTelemetry-compatible request tracing

[SisyphusZheng]

Summary

Add OpenTelemetry-compatible request tracing to the openElement request pipeline. SvelteKit (2025) shipped native OTel tracing as an experimental feature; openElement should provide equivalent observability for production operations.

Competitive context

Framework	Observability
SvelteKit	Native OTel tracing (experimental, 2025): auto-traces handle hooks, load functions, form actions
Astro	No built-in
Next.js	No built-in (Vercel provides proprietary)
Fresh	No built-in
openElement	reportError telemetry hook only — no request tracing

Proposed design

// instrumentation.server.ts (new convention)
import { initTracing } from '@openelement/app/tracing';

export default initTracing({
  serviceName: 'my-app',
  // OTel SDK config passthrough
});

Auto-traced spans:

• loader execution (route path, duration, status)
• action execution (route path, method, status, validation outcome)
• DSD render (component tag, duration, nesting depth)
• SSG prerender (page count, total time)

Scope

• @openelement/app/tracing subpath (optional import, zero cost when unused)
• Span emission via OTel SDK protocol (not bundled — peer dependency)
• Dev-mode: console-friendly span logging
• Prod-mode: OTLP export

Non-goals

• No bundled OTel SDK (peer dep or user-provided)
• No metrics/gauges (traces only for v1)
• No dashboard (user brings Grafana/Datadog/etc.)

Target version

0.44.0 (Production Runtime)