test(trace): add legacy characterization replay fixtures

tests/fixtures/trace-v1/README.md

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+# Trace v1 characterization fixture
+
+`speedtests.jsonl` is an observed prefix of a live legacy-controller capture
+recorded during speed and LibreQoS testing. It preserves the original records,
+sequence values, and order through evaluation 150. The prefix intentionally has
+no synthetic `end` record and may include undesirable legacy behavior; it is an
+extraction-characterization artifact, not a correctness oracle.