fix(quickadapter): use directional PnL momentum gate (#149)

fix(quickadapter): use directional PnL momentum gate (#149)

Replace the velocity/acceleration t-statistic exit gate with a deterministic
mean per-candle PnL velocity direction rule: once the take-profit target is
reached and the recent window is complete, allow the exit iff recent[-1] <
recent[0] (equivalent to strictly negative mean velocity via telescoping).

Remove the now-dead machinery: decline_quantile, t-critical/effective-df,
acceleration and zero-variance fallbacks, get_pnl_momentum,
get_trade_unrealized_pnl_history, the thresholds_calibration config specs, and
the unrealized_pnl_timeframe_minutes field. A legacy
exit_pricing.thresholds_calibration setting now warns as obsolete and ignored.

Fail-open only where the horizon is unmeasurable (missing candle date,
incomplete window, non-finite value in the selected window); complete finite
windows are deterministic (falling exits; flat or rising blocks).

Fixes #128