[xiehuanyi]

For BaseDict[str, int], get("missing", "fallback") works at runtime but is rejected by the type checker. pop("missing", None) is incorrectly typed as int even though it returns None.

Add overloads that match native dictionary fallback typing. Cover the result types and runtime values for both a wrapped dictionary and native storage. All 964 unittest cases pass. Package and focused type checks, Ruff lint, and formatting pass.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.69%. Comparing base (c4c8a58) to head (a78fd9a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #632   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          65       65           
  Lines        2446     2447    +1     
=======================================
+ Hits         2414     2415    +1     
  Misses         32       32           

Flag	Coverage Δ
unittests	98.69% <100.00%> (+<0.01%)	⬆️

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