Track LGV formalization outside RealRooted

[PerAlexandersson]

The Lindström–Gessel–Viennot layer and the related non-nesting-rook development are intentionally on ice and are not needed for the current A262704 closure.

Preferred eventual organization:

• keep LGV code completely disjoint from RealRooted, ideally in a separate Lean repository/package so its dependency graph and builds stay small;
• do not port or merge the pending LGV-layer move into RealRooted now;
• leave the non-nesting-rook project untouched until this issue is explicitly resumed;
• if a future RealRooted theorem needs LGV, depend only on a small published interface rather than importing the full development.

This issue records scope and repository ownership only. No LGV formalization is currently requested.