Propagate trace metadata through TraceLens analyzers

[ajassani]

Summary

• Preserve top-level trace JSON fields as trace-level metadata while keeping traceEvents as the event payload.
• Expose metadata through TraceToTree and TreePerfAnalyzer for report consumers.
• Keep metadata optional so direct synthetic-event construction remains backward compatible.

Test plan

• python -m pytest tests/test_trace_metadata.py
• python -m pytest tests/test_trace_metadata.py tests/test_kernel_launchers.py -q