Rename MatchEngine::add_order to MatchEngine::execute

[SuperFluffy]

Summary

Rename MatchEngine::add_order to MatchEngine::execute for better API clarity.

Rationale

• The method doesn't just "add" orders - it executes them (fills, placements, rejections)
• Matches the underlying Book::execute naming
• "Execute" is standard terminology in trading systems

Implementation

Simple method rename in src/engine.rs and update tests. Breaking change but minimal impact.