Recover corrupt versioned stores instead of crashing

[xxfast]

Decoding a corrupt or partially written versioned store crashes instead of falling back to the migration. VersionedCodec.decode re-reads the version file and the raw data inside its own catch block with nothing guarding either, so an empty or truncated file throws straight back out. Reported three times: #80, #157, #162.

A missing version file still reads as 0 like before, an unreadable one now reads as null, which Migration<T> already allows for. Nothing that works today changes behaviour.

Also ports the moveOrCopy fallback from #159 into encode, which still called atomicMove directly and so never got fixed for versioned stores on Android 7 and below.

• Fix KVersionedStore to handle corrupt or partially written files gracefully

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.26%. Comparing base (6a52682) to head (c71362e).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   92.68%   94.26%   +1.57%     
==========================================
  Files           8        8              
  Lines         123      122       -1     
  Branches       12       11       -1     
==========================================
+ Hits          114      115       +1     
+ Misses          1        0       -1     
+ Partials        8        7       -1     

Files with missing lines	Coverage Δ
...b/xxfast/kstore/file/extensions/KVersionedStore.kt	100.00% <100.00%> (+5.26%)	⬆️