Formalize action constants and remove Uint from codec interfaces

[cdvelop]

Executed the implementation plan in docs/PLAN.md:

1. Formalized CRUD action bytes as plain byte constants (ActionCreate, ActionRead, ActionUpdate, ActionDelete) to serve as a single source of truth.
2. Replaced all magic literals ('c', 'u', 'd') in the validation logic and documentation.
3. Simplified the codec contract by removing the vestigial Uint methods from FieldWriter and FieldReader, aligning with the project's 64-bit-only design.
4. Updated the mock implementations and test suite to reflect these changes.
5. Updated all relevant documentation files.

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PR created automatically by Jules for task 17389871191021151875 started by @cdvelop

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