Ehrhart's volume conjecture

### What is the conjecture
OpenAI reports a proof concerning Ehrhart's volume conjecture, accompanied by a Lean 4 formalization.

This issue is to request that the result be considered for addition to Formal Conjectures as `@[category research solved]`. Please see the linked official sources for the exact statement and scope.

Official article: https://openai.com/index/ten-advances-in-mathematics/
Paper: https://cdn.openai.com/pdf/ten-proofs-oai.pdf
Lean proof at an immutable commit: https://github.com/openai/ten-proofs/blob/a13547c6be4563746881d0b3b4c9fd03f72f0484/EhrhartVolumeInequality.lean#L55737-L55753
Comparator statement template: https://github.com/openai/ten-proofs/blob/a13547c6be4563746881d0b3b4c9fd03f72f0484/ComparatorChallenges/F_EhrhartVolumeInequality.lean

### Prerequisites needed
The linked Comparator Challenges file provides a compact Lean statement template. The full proof is external and can be referenced with `@[formal_proof using lean4 at "..."]`.

### [AMS categories](https://github.com/google-deepmind/formal-conjectures/labels?q=ams-)

* ams-52

### Choose either option
- [ ] I plan on adding this conjecture to the repository
- [x] This issue is up for grabs: I would like to see this conjecture added by somebody else

Start with ComparatorChallenges/F_EhrhartVolumeInequality.lean and compare its statement template with the referenced Lean proof at the immutable commit. Add the conjecture to Formal Conjectures with the stated research-solved category and reference the external Lean 4 proof; done means the repository accepts the formalized statement.

Written by the indexing model from the issue text.