feat(traces): span handles (#951)

* feat(traces): span handles

Adds the Span interface (posthog.tracing.span, the only public module in the
package) and its handles. The recording span keeps its timing on a monotonic
clock, with children of a local parent on the root's clock basis so they stay
inside it; end() is idempotent and hands the pipeline one record. The no-op
handle is returned when tracing cannot run, and the pass-through handle echoes
an inbound traceparent and tracestate so a service with tracing off still
forwards the trace, including from spans nested inside it. Entering a handle
makes it the active span for the block and records an exception raised out of
it. Not reachable from the client.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TkZAsCciW4PV8ZdcCHmAbA

* fix(traces): detach before ending and record only Exception on scoped exit

A generator closed early or a cancelled task raised GeneratorExit or
CancelledError through __exit__ and shipped as an error span. The scoped
form now records an Exception only, still ending on every exit. The span
is detached before end() so nothing started from the on_end path nests
under a span that is over. end() flips its flag under the handle's lock,
and a deactivate that finds no token for the context says so at debug.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: turnipdabeets