Lean formalization of Generic Rigidity and the Grassmannian

Lean 4 formalization accompanying Alexander Heaton's paper Generic Rigidity
and the Grassmannian (https://arxiv.org/abs/2604.05442). The release contains the formalized main
results, including the balancing/self-stress equivalence and supporting
stream, path-system, and tableau arguments. It includes the Lean source,
pinned Lean 4.28.0 and mathlib v4.28.0 configuration, build instructions, and
a correspondence between the paper and the formal statements.

The Asimow-Roth equivalence at generic placements and the existence of
generic placements remain explicit hypotheses in the statements that use
them. The paper's illustrative worked examples are not individually
formalized. The proofs were developed with assistance from Aristotle
(Harmonic).