Switch zstandard library to stdlib compression.zstd or backports.zstd

[yunzheng]

fixes #197
supersedes #196

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.27%. Comparing base (b0cb20a) to head (3c9940a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
flow/record/base.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #198      +/-   ##
==========================================
- Coverage   83.42%   83.27%   -0.15%     
==========================================
  Files          35       35              
  Lines        3699     3696       -3     
==========================================
- Hits         3086     3078       -8     
- Misses        613      618       +5     

Flag	Coverage Δ
unittests	83.27% <85.71%> (-0.15%)	⬇️

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[Copilot]

Pull request overview

This PR migrates from the third-party zstandard library to the stdlib compression.zstd (Python 3.14+) or its backport backports.zstd (Python <3.14). The changes simplify the API calls and remove the PyPy restriction for zstd compression support.

• Replaces zstandard with conditional import of compression.zstd (Python ≥3.14) or backports.zstd (Python <3.14)
• Updates zstd API usage from separate compressor/decompressor contexts to unified ZstdFile interface
• Removes PyPy restriction for zstd compression support

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pyproject.toml	Updates dependency from zstandard to backports.zstd with Python version constraint
flow/record/base.py	Implements conditional import logic and updates API usage from context-based compression to ZstdFile
tests/record/test_adapter.py	Updates skip message and removes PyPy restriction test skip

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