CATTRI-1: Fixed expose: :read preventing internal writes

[bnlucas]

• Added regression for expose: :read internal assignment (spec/cattri_spec.rb).
• Adjusted writer logic to generate internal writers for read-only exposure (lib/cattri/attribute.rb) and updated allowed method expectations (spec/cattri/attribute_spec.rb).
• Added coverage for skipped writer branch by stubbing writable? to false in the compiler spec (spec/cattri/attribute_compiler_spec.rb).
• Ensured read-only writers remain private but callable internally and updated predicate/allowed methods grids accordingly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (122638f) to head (8800af4).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #9   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          378       378           
  Branches        59        59           
=========================================
  Hits           378       378           

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