fix(cpp14): replace d2x.harness with d2x in 00-generic-lambdas exercise

d2x.harness module does not exist in d2x/src/. The exercise import
was failing at module resolution before reaching any exercise code.
Changed to import d2x which is the existing module, consistent with
the exercise template in .agents/skills/d2mcpp-authoring/assets/exercise.cpp.

Author: lczllx