[stable32] chore(deps): point stable32 to pdf validator backport

[vitormattos]

Summary

• point the 3rdparty submodule to the PR-backed stable32 vendor commit from [stable32] feat: add libresign/pdf-signature-validator to scoped vendor 3rdparty#75
• replace the orphan/local submodule SHA currently referenced on stable32 with the published backport commit 6668324
• keep the native PDF validation backport available on stable32 without downgrading the package

Validation

• runtime smoke tests for libresign/pdf-signature-validator passed on PHP 8.1 and PHP 8.3 against signed and unsigned fixture PDFs
• an equivalent package unit suite passed on PHP 8.1 with 55 tests, 168 assertions, and 3 environment-only skips
• the same equivalent package unit suite produced the same result on PHP 8.3

Depends on LibreSign/3rdparty#75.