Add market depth and kline websocket subscriptions

[zaixincheng174-ai]

/claim #91
/claim #92

Closes #91
Closes #92

Summary

• add typed market depth and K-Line websocket data types to the market socket event model
• expose marketDepth$ and kline$ streams on BingxMarketSocketStream
• add subscribeMarketDepth(...) and subscribeKline(...) helpers while preserving the existing generic subscribe(...) API
• export the new public market websocket types
• mark the README market depth and K-Line websocket features as supported

Validation

• npx jest src/bingx-socket/bingx-market-socket-subscriptions.spec.ts --runInBand
• npx eslint src/bingx-socket/bingx-market-socket-stream.ts src/bingx-socket/events/market-websocket-events.ts src/bingx-socket/events/index.ts src/bingx-socket/bingx-market-socket-subscriptions.spec.ts
• npx prettier --check src/bingx-socket/bingx-market-socket-stream.ts src/bingx-socket/events/market-websocket-events.ts src/bingx-socket/events/index.ts src/bingx-socket/bingx-market-socket-subscriptions.spec.ts
• git diff --check
• npm run build

Disclosure

Prepared with AI assistance and locally validated.

[zaixincheng174-ai]

Follow-up pushed in 3fbb8f6: aligned the market depth stream name with the current BingX swapV2 websocket docs. The docs list depth subscriptions as <symbol>@depth<level> (for example BTC-USDT@depth5, BTC-USDT@depth20, BTC-USDT@depth100), so I removed the unsupported interval suffix from subscribeMarketDepth(...) and its public depth data type. K-Line remains unchanged because swapV2 docs use BTC-USDT@kline_1m and the existing interval union matches that format.

Validation:

• npm test -- --runInBand --forceExit bingx-market-socket-stream.spec.ts bingx-market-socket-subscriptions.spec.ts
• npm run build
• npx eslint src/bingx-socket/bingx-market-socket-stream.ts src/bingx-socket/events/market-websocket-events.ts src/bingx-socket/events/index.ts src/bingx-socket/bingx-market-socket-subscriptions.spec.ts
• git diff --check