feat(ws): add resilient websocket options

[SebastianBoehler]

Summary

• add WebSocketOptions, WebSocketStats, typed stream message callbacks, and subscription tracking to WebSocketClient
• add bounded message dispatch with deterministic drop-oldest backpressure and metrics
• configure reconnect backoff/ping behavior through IXWebSocket and restore tracked subscriptions after reconnect
• update OrderbookManager to track and restore its subscription message without changing its existing raw callback behavior
• add deterministic tests for queue overflow, typed message parsing, ignored heartbeat messages, and malformed JSON parse handling
• document the new WebSocket resilience API in the README

Validation

• cmake -S . -B /tmp/polymarket-cpp-client-verify -DPOLYMARKET_CLIENT_BUILD_EXAMPLES=ON -DPOLYMARKET_CLIENT_BUILD_TESTS=ON
• cmake --build /tmp/polymarket-cpp-client-verify -- -j4
• ctest --test-dir /tmp/polymarket-cpp-client-verify --output-on-failure
• git diff --check