Thirdparty compression deps

[ColinLeeo]

Summary

This PR introduces bundled source subsets for stable C++ codec/compression dependencies:

• Zstandard 1.5.7
• XZ Utils 5.8.3 / liblzma

The intent is to record the third-party source provenance first, before wiring these libraries into the C++ build or TsFile runtime logic.

Changes

• Add cpp/third_party/zstd-1.5.7 source subset
• Add cpp/third_party/xz-5.8.3 source subset
• Add cpp/third_party/README.md to record upstream source and version
• Add per-dependency README files describing retained source subsets
• Update root LICENSE with bundled third-party information

Notes

This PR only introduces third-party source files and license/source metadata. It does not change C++ build behavior or TsFile runtime behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.52%. Comparing base (86252e5) to head (5d4ec08).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #859   +/-   ##
========================================
  Coverage    60.52%   60.52%           
========================================
  Files          735      735           
  Lines        48439    48439           
  Branches      7679     7679           
========================================
+ Hits         29319    29320    +1     
  Misses       17718    17718           
+ Partials      1402     1401    -1     

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