Make JsonLogger writes asynchronous

## Context

`JsonLogger` (core/utils/JsonLogger.h) serializes every write behind `m_mutex` and writes synchronously to an `std::ofstream`. Any thread that logs, including strategy and market data paths when `--json-log` is enabled, pays mutex contention plus file I/O on the calling thread. The docs were recently corrected to say exactly this (PR #73), but the better fix is removing the overhead.

## What to do

- Move serialization and file I/O to a dedicated writer thread fed by a bounded queue (an SPSC or MPSC ring buffer fits the existing lock-free style of the codebase).
- Define overflow behavior explicitly (drop with a counter, or block) so the hot path never does unbounded work.
- Flush and drain the queue on shutdown so no events are lost on clean exit.
- Add a benchmark comparing log-call latency before and after.

## Notes

After this lands, update the JSON Data Export doc's thread-safety wording again to describe the async design.

Start with core/utils/JsonLogger.h and inspect its mutex, ofstream, and existing write path, then review the codebase's lock-free queue style. Define bounded-queue overflow behavior, asynchronous serialization and file I/O, and clean shutdown draining; add the requested before/after log-call latency benchmark. Afterward, update the JSON Data Export documentation wording to describe the async design.

Written by the indexing model from the issue text.