feat(traces): OTLP span encoding and client-side validity

Adds the OTLP JSON encoder for spans: AnyValue encoding per the traces spec
(int64 as strings, out-of-range ints and non-finite floats as strings, None
values and empty keys dropped, unpaired surrogates replaced with U+FFFD, a
bounded walk that terminates on cycles), the span record builder with the W3C
flags byte and OTel remoteness bits, the one-resource/one-scope envelope with
service.name always present, and the name and timestamp sanitizers. One bad
value otherwise gets the whole batch rejected. Not reachable from the client.

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

Author: turnipdabeets