ENH: Support time-varying bid-ask spreads

[AtomicGlance]

Summary

Fixes #1300.

Backtest(..., spread=...) now accepts either the existing scalar rate or a one-dimensional per-bar array-like value. A pandas Series must use the same index as the OHLC data. Values are validated as finite, numeric, and non-negative, and are applied at the bar where an order is filled. Scalar spreads keep the existing execution path and results.

The spread remains a relative rate, consistent with the current API. Data-source spreads expressed in price units can be converted to rates before passing them to Backtest.

Tests

• scalar and constant Series spreads produce identical trades and equity curves
• variable spreads affect market, limit, and stop fills at the fill bar
• long and short fills use the correct direction
• invalid length, shape, type, index, NaN, infinite, and negative values are rejected
• full backtesting/test/_test.py suite: 83 passed, 1 skipped