Add SpanContext::toTraceparent()

[mar-cf]

No description provided.

[ask-bonk]

I've completed my analysis. Let me summarize the review:

The code is clean, correct, and well-tested. Let me verify my analysis:

1. toTraceparent() correctly returns kj::none for all invalid states (no spanId, no traceFlags, zero traceId, zero spanId).
2. formatW3CTraceparent() correctly produces W3C-compliant format with proper zero-padding via addHex() (16 chars per 64-bit field) and kj::encodeHex() (2 chars for flags byte).
3. Tests cover round-tripping, zero-padding, and all kj::none return paths.
4. No behavioral changes to existing code — this is purely additive.
5. No backward compat concerns — new methods only.
6. Memory safety — no lifetime issues, no async patterns, no V8 interaction.

The one minor observation: formatW3CTraceparent is declared as a public free function in the header but is currently only called from SpanContext::toTraceparent(). It's reasonable to expose it for reuse by InvocationSpanContext or other callers in the future, so this is a deliberate design choice, not a problem. The PR title mentions the branch is cache-user-tracing, suggesting more callers are coming.

LGTM

github run

[codspeed-hq]

Merging this PR will degrade performance by 9.19%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 71 untouched benchmarks
⏩ 129 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	bm_Promise_Fib10	18.5 µs	20.3 µs	-9.19%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing cache-user-tracing (20e92f4) with main (3d43d41)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩