[MINOR] Fix class-init deadlock between AOffset and its subclasses

[Baunsgaard]

AOffset initialized a cached empty slice in its static initializer by instantiating its own OffsetEmpty subclass. Since OffsetEmpty (and the other offset subclasses) depend on AOffset being initialized first, this formed a superclass/subclass class-initialization cycle. When two threads first touched the offset classes concurrently (e.g. parallel test execution), each could hold one class's init monitor while waiting for the other, deadlocking on the JVM class-initialization monitors. Such a deadlock is invisible to the JVM deadlock detector and cannot be interrupted, so the affected JVM hangs indefinitely. It only manifests under concurrent first-touch, which is why it never reproduced in single-threaded local runs.

Defer the empty slice to a lazy holder accessed via emptySlice(), so AOffset's static initializer no longer references any subclass. By the time the holder is touched, AOffset is already initialized, so no cycle exists.

Add a regression test that forces concurrent first-initialization of the offset classes through a dedicated class loader across repeated rounds and fails if it does not complete promptly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.42%. Comparing base (e4f0987) to head (a88db67).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2502      +/-   ##
============================================
- Coverage     71.47%   71.42%   -0.06%     
+ Complexity    48883    48848      -35     
============================================
  Files          1573     1573              
  Lines        189238   189239       +1     
  Branches      37128    37128              
============================================
- Hits         135261   135167      -94     
- Misses        43530    43609      +79     
- Partials      10447    10463      +16     

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