feat(mojo): add mojo-json client from leo-gan/gld-json

[leo-gan]

Summary

• Add a mojo-json harness client for leo-gan/gld-json 0.2.0
• Vendor sources as gldjson_* so they can share a pixi env with mojo-avro
• Register the serializer in benchmark_config.yaml and every experiment that already lists EmberJson
• Refresh Mojo dashboard + experiment results from full run 2026-09-09-131928

Validation

• Tests: analysis 90 passed; python 32 passed; javascript 10 passed; rust 15 passed; pixi run test in mojo/ printed ok
• Benchmarks: full, langs mojo, stem 2026-09-09-131928 (7000 rows, fidelity 1.0 for all 7 serializers including mojo-json)
• Experiments: timed 14 folders for mojo
• Error CSVs: clean (0 error rows)
• Analysis: reports/mojo snapshot from that stem; docs/mojo/index.md lists the new row
• Dashboard: sync-data.py → mojo_latest.json.gz + stats_mojo_latest.json.gz + available_runs.json + experiment catalog (other language payloads left unchanged)

Notes

• config/benchmark_config.yaml only adds a Mojo serializer. Full bench was scoped with PREPARE_PR_LANGS=mojo rather than re-running every language.
• The client uses vendored WireWriter / WireReader expected-order encode/decode of the suite types. Package names are rewritten because mojo-avro already owns json / runtime / wire.