[songgaoye]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:

Base64 encoding and decoding use scalar implementations on x86 even when AVX2 is available, limiting throughput for medium and large payloads.
What is changed and the side effects?
Changed:

• Add an AVX2 Base64 codec derived from aklomp/base64, with BSD-2-Clause attribution.

• Dispatch to AVX2 only on supported x86 CPUs.

• Use measured thresholds to retain the scalar path for small inputs:

  • Encode: AVX2 from 32 bytes.
  • Decode: AVX2 from 256 bytes of Base64 text.

• Add unit and fuzz coverage for binary round trips and invalid input.

• Add reproducible scalar and AVX2 benchmark targets via BUILD_BASE64_BENCHMARKS.
  Side effects:

• Performance effects:

  • On an Intel i7-10700, AVX2 improves encode throughput by roughly 5–12x and decode throughput by roughly 1.4–4.7x for sufficiently large inputs.
  • Small payloads continue to use the scalar path.

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

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