Give each codec its own staging file

[xxfast]

Supersedes #154, same issue (#85).

Two stores on one file share a staging path ($file.temp), so concurrent writes can interleave into one buffer. Each codec now stages through its own, so the move stays atomic and the worst case is last write wins.

#154 deduplicated stores in a global registry instead. That changes what storeOf means, and can't be keyed reliably since SystemFileSystem.resolve throws when the file doesn't exist yet, which is exactly the case at storeOf time.

Not a confirmed cause of #80, #157 or #162, those look more like #175. Sync half of #85 is still #124.

Kept @Khudoyshukur's commit.

• Reuse KStore instance if the same codec is provided
• Refactor FileCodec to use unique temporary files for staging and simplify store creation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.35%. Comparing base (fdc1b10) to head (b62faf9).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   94.26%   94.35%   +0.09%     
==========================================
  Files           8        8              
  Lines         122      124       +2     
  Branches       11       11              
==========================================
+ Hits          115      117       +2     
  Partials        7        7              

Files with missing lines	Coverage Δ
...n/kotlin/io/github/xxfast/kstore/file/FileCodec.kt	100.00% <100.00%> (ø)
...b/xxfast/kstore/file/extensions/KVersionedStore.kt	100.00% <100.00%> (ø)