Refactor internals: make callsite helpers private, rename sentinel and error classes, add __all__

[Code0x58]

Motivation

• Reduce public surface and clarify intent by making caller-path helpers private and limiting exports via __all__.
• Improve naming consistency for sentinel and exception types to be more explicit (_UNDEFINED_VALUE, AddTooLateError, MultipleDefaultsError, CrossAlternativesImplementationError).

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0acc5be) to head (99e1494).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          194       195    +1     
=========================================
+ Hits           194       195    +1     

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