audit: Token sale audit report (#518)

* audit: add token sale audit report for release v1.5.1

* docs: add audits/README report entry

* docs: update label

Author: 0xNeshi

audits/2026-07-v1.5.1-token-pre-sale.pdf

@@ -17,3 +17,4 @@
 | July 2026  | v1.5.0  | [`55a7971 → bb2405c`](https://github.com/OpenZeppelin/contracts-sui/compare/55a79713a7e51a142de017e57bd7901ce8198212...bb2405c2b04165854813dc2a06683a82a5d1e620) | OpenZeppelin | `openzeppelin_fp_math` Gaussian functions and fixed-point API diff | [🔗](./2026-07-v1.5.0-gaussian-fp-math.pdf) |
 | July 2026  | v1.5.0  | [`6fcf34c`](https://github.com/OpenZeppelin/contracts-sui/tree/6fcf34ce15fd6ec7770c331b6918e9574e09505d) | OpenZeppelin | `openzeppelin_sale` Token Pre-Sale modules | [🔗](./2026-07-v1.5.0-token-pre-sale.pdf) |
 | July 2026  | v1.5.0  | [`63e1572`](https://github.com/OpenZeppelin/contracts-sui/tree/63e1572747a360d45f99d022c8076e8bedf5a26f) | OpenZeppelin | `collections` SortedMap and SortedSet modules | [🔗](./2026-07-v1.5.0-data-structures.pdf) |
+| July 2026  | v1.5.1  | [`6fcf34c`](https://github.com/OpenZeppelin/contracts-sui/tree/6fcf34ce15fd6ec7770c331b6918e9574e09505d) | OpenZeppelin | `openzeppelin_sale` audit fixes | [🔗](./2026-07-v1.5.1-token-pre-sale.pdf) |