Summary

• Keep modified backtest orders leaves_qty in sync with the requested qty.
• Restore leaves_qty if a local modify request is rejected before reaching the exchange.
• Add a regression test for a requeued modify that increases order quantity and then fills.

Root Cause

Local::modify and L3Local::modify updated qty but left leaves_qty unchanged. When L2 exchange processors handled a modify by canceling and re-adding the order, ack_new reused the stale leaves_qty, so the order could still execute with the old remaining quantity.

Validation

• cargo test -p hftbacktest modifies_order_leaves_qty_when_requeued
• cargo test -p hftbacktest --lib

Note: cargo test -p hftbacktest still fails on existing example compile errors unrelated to this change.