Capture and surface registration call-site labels for Implementations (debug mode)

[Code0x58]

Motivation

• Improve diagnostics when debug mode is enabled by recording where each Implementation was registered so errors and repr(...) output can disambiguate which implementation instance is involved.
• Document the new label behavior in the README.md to make the debug information discoverable.

Description

• Add a label: str | None field to Implementation and populate it from maybe_get_caller_path() when implementations are created or duplicated via Alternatives.add.
• Provide a __post_init__ fallback to capture the caller path if the label was not supplied, and update Implementation.__repr__ to include the label when present.
• Ensure the Alternatives constructor and add path pass caller-derived labels into new Implementation objects and include the label in some debug messages.
• Update README.md to mention that Implementation captures a registration call-site label in debug mode, and add two tests to test_alternative.py that verify label population and the safe fallback when caller info is unavailable.

Testing

• Added and ran test_implementation_label_populated_in_debug which verifies labels include caller metadata when DEBUG is enabled, and it succeeded.
• Added and ran test_implementation_label_safe_when_caller_unavailable which verifies the fallback label is used when caller info cannot be resolved, and it succeeded.
• Ran the full pytest suite and the tests completed successfully.

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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 (868175b) to head (c1092eb).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #8   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          189       199   +10     
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+ Hits           189       199   +10     

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