feat: local observability playground

[julio4]

builder-playground start dev/playground.yaml now launch grafana and jaeger automatically.
Dedicated ./dev directory for local env, where grafana dashboards/contender scenarios can live.

Other changes:

• Move payload/flashblock tracing to tracing::Instrument for async-safe span propagation.
• Validate OTLP HTTP endpoint before creating telemetry layer.
• Add a macOS-specific panic hook workaround for OpenTelemetry TLS teardown panic issue.
• Move playground config to dev/playground.yaml and add Prometheus + Grafana datasource provisioning for local observability.