Let's Fix Hashing Again (Damn)

[Sewer56]

Simple PR which:

• Fixes hashing of chunked blocks by changing XxHash64Algorithm _hash = new(); to XxHash64Algorithm _hash = new(0);
• Adds missing asserts which compare Nx header's hash values against the hash value of extracted data, an oversight on my part.
• Fixed very rare bug where thread pre-emption by the OS (under high load) would cause chunked blocks (with multiple chunks) to produce the wrong hash.
  • Caused by out of order calls to AppendHash and GetFinalHash.
    • Calls to these methods are now synchronized (thread safe) for ChunkedBlockState.
      • Has no measurable impact on performance in benchmarks. Due to how library pipelines blocks for compression.
    • No change needed for SolidBlock(s).
  • Found by stress testing over 30+ minutes of time on the SMIM dataset on all threads.
    • CPU load needed to be ~100% to reproduce.
• Added a stress test against SMIM which you can manually run (if desired).
  • This is disabled by default with a Skip test directive, but can be re-enabled if needed.
  • The SMIM dataset uses all of Nx' block types (Solid, Chunked with 1 Chunk, Chunked with Multiple Chunks).
  • Hashes compared against in this stress test were made by xxh64sum, the official binary.
• Bumps xxHash64 dependency.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (b2a1cef) 91.26% compared to head (fdfefa9) 91.51%.
Report is 10 commits behind head on main.

Files	Patch %	Lines
NexusMods.Archives.Nx/Utilities/Compression.cs	50.00%	5 Missing ⚠️
...ods.Archives.Nx/Structs/Blocks/ChunkedFileBlock.cs	80.00%	1 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   91.26%   91.51%   +0.25%     
==========================================
  Files          42       41       -1     
  Lines        1374     1297      -77     
==========================================
- Hits         1254     1187      -67     
+ Misses        120      110      -10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.