Use C++17 concatenated namespaces, add [[nodiscard]] to query methods,
add missing direct includes, and use pass-by-value + std::move in
NullTelemetry constructor.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Redesign SpanGuard with pimpl idiom to hide all OpenTelemetry types
from public headers. Add global Telemetry accessor so SpanGuard factory
methods work without explicit Telemetry references. Add child/linked
span creation and cross-thread context propagation. Update plan docs
to reflect macro removal in favor of SpanGuard factory pattern.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>