Preserve FIFO time priority after order cancellation

[Hei-MaoM]

Fixes #3

CancelOrder used swap-and-pop for non-tail cancellations. That keeps the
index map consistent but moves the newest order ahead of older orders at the
same price, violating the documented price-time (FIFO) priority.

This change:

• erases the cancelled order in place so surviving orders keep arrival order;
• re-indexes the shifted suffix in the order lookup map;
• adds a regression test that cancels the head order and verifies the next
  resting order matches first.

The order-preserving erase changes cancellation from O(1) to O(n) in the
number of orders after the cancelled slot, which is required to keep FIFO in
the existing contiguous-vector representation.

Validation:

• g++ -std=c++20 -O2 -Wall -Wextra -pedantic tests.cpp
• OrderBookTests: all 22 functionality tests passed
• Performance benchmarks and high-frequency simulation completed successfully