BMv2 packet trace support

BMv2 currently provides only textual logs for tracing packets through a P4 pipeline, which must be parsed with brittle regex hacks and cannot be easily consumed by automated tools. This project aims to replace that with a structured, programmatic trace API — recording per-packet events such as applied tables, matched entries, and executed actions — and exposing them in a machine-readable format such as protobuf (with JSON serialization support), enabling use cases ranging from human debugging to automated test coverage measurement, test generation, and root cause analysis.