Fix typing transparency for alternative decorators

[Code0x58]

Summary

• fixed typing-related issues in alternative.py so decorator-based APIs remain type-checker transparent while preserving runtime behavior
• replaced direct method rebinding assignments with setattr in Alternatives.callable and Implementation.__call__ to satisfy type checkers without changing dispatch semantics
• removed duplicate attribute initialization and renamed nested decorator helper functions to avoid redefinition errors
• added a targeted cast in measure sorting to make comparator typing explicit

Validation

• uv run --dev ruff format .
• uv run --dev ruff format --check .
• uv run --dev mypy .
• uv run --dev pytest

Notes

• pyrefly binary is not available in this environment (uv run --dev pyrefly check . fails with No such file or directory).

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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 (c9e48d7) to head (7577ece).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #14   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          195       196    +1     
=========================================
+ Hits           195       196    +1     

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[Code0x58]

👀 this is just a temporary hack to get the pyrefly into the CI pipeline. mypy already passes so that is probably good enough. I'll get this looked into outside of Codex cloud, as that doesn't have pyrefly in the current environment