Mark eqSystem6_no_solution_d3 as solved via external Lean 4 proof

A complete external Lean 4 proof is now available for MonochromaticQuantumGraph.eqSystem6_no_solution_d3.

Pinned proof:
https://github.com/algal/krenn-gu-6x3-certificate/blob/c04696e515e0c02be140353fb52ea60c62e827b1/KrennGuCertificate/Unrestricted.lean#L201-L222

Repository and verification instructions:
https://github.com/algal/krenn-gu-6x3-certificate/tree/c04696e515e0c02be140353fb52ea60c62e827b1

The proved proposition is exactly the right-hand side of the existing benchmark theorem:

¬ ∃ W : MonochromaticQuantumGraph.WeightsN 6 3 ℂ,
MonochromaticQuantumGraph.EqSystemN 6 3 W

A clean clone of the pinned release passed all 50 committed artifact checksums and all 8,421 Lean build jobs. The final axiom report is:

[propext, Classical.choice, Lean.ofReduceBool, Lean.trustCompiler, Quot.sound]

There is no sorryAx or custom axiom in the proof. The Lean.ofReduceBool and Lean.trustCompiler dependencies arise from native_decide and are documented in the repository trust boundary.

Following CONTRIBUTING.md, I plan to submit a small status PR that:

- changes category research open to category research solved;
- replaces answer(sorry) with answer(True);
- adds a formal_proof using lean4 attribute pointing to the pinned proof above.

The long proof remains in its external repository, as prescribed by the formal_proof mechanism.

AI-use disclosure: the proof and this registration contribution were developed collaboratively with OpenAI Codex. Its correctness claim rests on the committed Lean verification and reproducible certificate checks, not on model-generated prose.

Read CONTRIBUTING.md and inspect the existing benchmark theorem and formal_proof conventions. Verify the pinned KrennGuCertificate/Unrestricted.lean proof using the repository instructions, then make the status, answer, and formal_proof metadata changes; done means the checks and Lean build pass without introducing a sorryAx or custom axiom.

Written by the indexing model from the issue text.