fix: interface compliance check for `API` struct

[brawncode]

updated the code to properly verify that API implements the da.DA interface. using var _ da.DA = (*API)(nil) ensures compile-time safety without any runtime overhead. all interface methods are correctly delegated to Internal, so this now reflects the intended design.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 35.41%. Comparing base (2469e7a) to head (1e3a2b7). :warning: Report is 582 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4488      +/-   ##
==========================================
- Coverage   44.83%   35.41%   -9.42%     
==========================================
  Files         265      304      +39     
  Lines       14620    24298    +9678     
==========================================
+ Hits         6555     8606    +2051     
- Misses       7313    14745    +7432     
- Partials      752      947     +195     

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