Every StartOrderSync spawned a ticker goroutine that ran forever — it
survived trader stop AND deletion, so each quick-created trader left a
permanent 30s Hyperliquid poll behind. Stacked leaks turned into an
~8s effective hammer that tripped Hyperliquid's 429 rate limit, which
then broke the symbol board, trader creation, and order sync itself.
- new trader/syncloop package: shared stoppable sync loop with
exponential failure backoff (30s base, 5min cap)
- all 9 exchanges' StartOrderSync now take the trader's stop channel
and stop when the trader stops (close broadcast from AutoTrader.Stop)
- provider/hyperliquid: GetPerpDexCoins now serves a 5min TTL cache and
falls back to the stale board when the upstream returns 429, so the
symbol panel keeps working through rate limiting