Fix CRC endianness detection and add s390x regression

[PaulTaykalo]

Summary

• switch CRC fast-path endianness detection to the existing Crypto/brg_endian.h logic
• stop relying on the undefined XAD_BYTE_ORDER_BIG_ENDIAN macro
• add a deterministic CRC regression binary
• run that regression in GitHub Actions under emulated linux/s390x

Testing

• added a dedicated CRC Big Endian GitHub Actions workflow for s390x
• local runtime validation is pending on CI because the emulated container build requires networked package install

[Copilot]

Pull request overview

This PR fixes CRC fast-path endianness detection by switching to the established Crypto/brg_endian.h byte-order logic, and adds deterministic regression coverage (including an emulated s390x GitHub Actions run) to prevent big-endian regressions.

Changes:

• Replace CRC endianness detection with PLATFORM_BYTE_ORDER == IS_BIG_ENDIAN from Crypto/brg_endian.h.
• Add deterministic fast-vs-slow CRC regression checks (XCTest + standalone regression binary).
• Introduce a GitHub Actions workflow that builds/runs the regression under emulated linux/s390x.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
CRC.m	Switches the big-endian conditional compilation to brg_endian.h’s PLATFORM_BYTE_ORDER detection.
XADMasterTests/CRCCalculationTests.m	Adds deterministic buffers and assertions to ensure fast CRC matches slow CRC for specific sizes/tails.
Tests/CRCFastRegression.m	Adds a standalone deterministic regression executable to compare slow vs fast CRC results.
.github/workflows/crc-big-endian.yml	Adds an emulated s390x CI workflow to compile and run the regression binary.

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[Copilot]

The workflow is restricted to the master branch for both push and pull_request, but the repository/PR diff base appears to be main (e.g., diff headers show index main..branch). If the default branch is main, this workflow won’t run for the PRs it’s meant to protect. Consider switching the branch filter to main, including both main and master, or omitting the branches filter and relying on the paths filter alone.

[Copilot]

fprintf uses %08x to print uint32_t values (slow/fast). uint32_t is not guaranteed to be an unsigned int on all platforms, so this can trigger warnings or incorrect output on some architectures/toolchains. Consider including <inttypes.h> and using PRIx32 (or casting to an explicitly matching type) for portable formatting.