0.9.0 — TV-verified margin-call model + lot-step quantity truncation

Changed (breaking)

Backtest results change for any margin-enabled or derived-quantity strategy
(the API is additive — one new optional setting). All three changes move the
broker onto TradingView's empirically verified behavior, validated against a
42-event TV v6 margin-call ledger (BINANCE:XAUUSDT.P 15m; every event's bar,
fill price, and liquidated quantity reproduced exactly, and the full 85-row
trade list replayed within one 0.001 lot step).

• Margin-call simulation matches TradingView's broker emulator.
  • Evaluation and fill happen once per bar at the bar's worst price for
    the position (low for longs, high for shorts) — previously the broker
    walked the 4-point intrabar path and could liquidate several times per
    bar at intermediate points.
  • The liquidated quantity follows TV's 10-step algorithm: the cover
    quantity is truncated to the symbol's minimum contract size (new
    StrategySettings.minQty, default 0.001, host-configurable) before the
    ×4 multiplier; when truncation yields zero the emulator liquidates
    exactly one unit (empirical TV behavior, undocumented by TV).
  • strategy.margin_liquidation_price rounds down to tick for longs, up
    for shorts (per the Help Center leverage article) instead of to the
    nearest tick.
• Derived order quantities truncate to the lot step. strategy.cash and
  strategy.percent_of_equity sizing now truncate the computed quantity to
  minQty, as TradingView does (equity 10067.60 @ 4740 → qty 2.123, not
  2.12396). Explicit and fixed quantities pass through untouched. Set
  minQty: 0 to disable the truncation entirely.

Added

• StrategySettings.minQty — the symbol's minimum contract size (lot
  step), the truncation unit for margin-call liquidations and derived order
  sizing. Exchange metadata, not a strategy() parameter: hosts configure it
  per symbol via broker.configure({ minQty }) or the EngineOptions.strategy
  override. Default 0.001.
• PortfolioSleeveSpec.minQty — per-sleeve lot step for
  PortfolioEngine, merged into each sleeve's strategy override.