fix(cpp14): replace d2x.harness with d2x in 00-generic-lambdas exercise (#86)

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

* 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

src/cpp14/tests/00-generic-lambdas/0.cpp

@@ -20,7 +20,7 @@
 //
 
 import std;
-import d2x.harness;
+import d2x;
 
 int main() {