Add fightertest sync test to engine CI/CD

# Task
Ahead of #2906, let's set up a seeded Beyond All Reason (BAR) fightertest within the engine repository to serve as the small-scoped first step of the eventual multi-platform sync testing.

# Context
This is a sub-task of the larger initiative to implement multi-platform CI sync testing ([Issue #2906](https://github.com/beyond-all-reason/RecoilEngine/issues/2906)). Establishing reliable sync testing is critical for cross-platform compatibility, and we need a robust underlying test to build the CI pipeline around.

Currently, a seeded BAR "fightertest" is the best and most comprehensive test we have ready to go for this purpose. By integrating this specific test into the engine repository, we establish a solid foundation for automated sync validation before expanding to more granular or specialized tests (like bulk float precision testing).

# Acceptance Criteria
- A seeded BAR fightertest is successfully integrated into the engine repository's testing suite.
- The test executes automatically on every PR and main branch commit.
- The test accurately validates sync state and fails the CI build appropriately if a desync occurs.
- Basic documentation is provided detailing how developers can run the fightertest sync test locally (pretty sure this exists somewhere but we should point to it)

Start by locating the existing seeded BAR fightertest and the engine repository's testing suite, then inspect the CI configuration used for PRs and main branch commits. Find the existing local-run documentation referenced in the issue. Done means the test runs automatically, detects a desync as a CI failure, and has documented local execution steps.

Written by the indexing model from the issue text.