parity(#143): post-migration span capture + diff vs #137 baseline

Captured from feature/tracing-migration f510940 (rebased on master 8284509),
same Jaeger / request set / DB snapshot procedure as pre-migration. Result:
names + semantic attributes preserved, traceparent intact, no orphaned spans,
async/streaming paths now single traces. See post-migration/README.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/tracing-baseline/post-migration/traces.json

@@ -0,0 +1 @@
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