add W3C Trace Context propagation to Envelope

[s3d-i]

Summary

Add an optional W3C Trace Context carrier to Envelope so Swift and Python consumers can join work into one distributed trace. Existing message_id and correlation_id remain protocol-level identifiers.

Proposed change

• Define a small trace-context message containing traceparent and optional tracestate.
• Add it as optional envelope metadata without changing existing field numbers.
• Treat missing or invalid context as absent; never reject an otherwise valid envelope.
• Regenerate the committed Swift and Python protobuf outputs.

Out of scope

• OTel manifest or custom semantic conventions.
• SDK instrumentation, exporters, Collector configuration, metrics, or logs.
• Creating one span per envelope or token event.
• W3C baggage propagation.

Acceptance criteria

• Existing envelopes remain wire-compatible.
• Context propagation behavior is documented.
• No OpenTelemetry runtime dependency is added to this package.
• npm run bootstrap:check passes.

Downstream Swift and Python instrumentation will be tracked separately.

References: OpenTelemetry context propagation · W3C Trace Context