Handle relational ARFF data during load

[cjdoris]

Summary

• reset the nested relational reader to use a fresh IOBuffer and clear chunk state before reading
• enable the relational fixture assertion now that loading succeeds
• document the relational reader reset requirement in AGENTS guidance

Testing

• julia --project=@. -e 'using Pkg; Pkg.test()' (passes)

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.92%. Comparing base (1c3a4f2) to head (c48f1a9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #28       +/-   ##
===========================================
+ Coverage   51.91%   81.92%   +30.00%     
===========================================
  Files           1        1               
  Lines         705      708        +3     
===========================================
+ Hits          366      580      +214     
+ Misses        339      128      -211     

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