feat: declare initVariant() as weak

[Aymane-ST]

Summary

Declare initVariant() as weak in ArduinoCore-API.

Why

Some cores provide a default initVariant() implementation while allowing variants or boards to override it. Marking the declaration as weak makes that extension pattern explicit and consistent at the API level.

Compatibility

This is backward-compatible:

• existing overrides keep working,
• cores that do not override initVariant() are unaffected,
• the change only clarifies the intended linkage behavior.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.53%. Comparing base (cd91833) to head (7e1cbd8).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #266   +/-   ##
=======================================
  Coverage   95.53%   95.53%           
=======================================
  Files          16       16           
  Lines        1076     1076           
=======================================
  Hits         1028     1028           
  Misses         48       48           

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