GASKIT-8: Alpha of v0.4.0, implementing Cattri backend

[bnlucas]

• Refactored Castkit internals to build on Cattri registries for attributes, configuration, contracts, and attribute definitions
• Added Cattri-powered introspection helpers, serialization toggle behavior and enhanced type/validator DSLs, plus coverage for the new plugin helpers

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a2b727f) to head (952ae69).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main       #19       +/-   ##
============================================
+ Coverage   71.42%   100.00%   +28.57%     
============================================
  Files           1        47       +46     
  Lines           7      1177     +1170     
  Branches        0       158      +158     
============================================
+ Hits            5      1177     +1172     
+ Misses          2         0        -2     

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