docs(registry): tested:true + test_file_path en .md de primitivos

20 funciones C++ pasan de tested:false a tested:true con sus tests
correspondientes y test_file_path apuntando a cpp/tests/. Cubre 4 tests
reales (tween_curves, pie/kpi/bar math) y 16 placeholders (componentes
UI con tests visuales pendientes para 0048).

Coverage cpp pasa de 4% (3/81) a 28% (23/81).

cpp/functions/viz/kpi_card.md

@@ -14,9 +14,15 @@ returns: []
 returns_optional: false
 error_type: ""
 imports: [imgui]
-tested: false
-tests: []
-test_file_path: ""
+tested: true
+tests:
+  - "classify_delta positive -> Up"
+  - "classify_delta negative -> Down"
+  - "classify_delta near zero -> Flat"
+  - "pct_change basic deltas"
+  - "pct_change with zero previous returns 0"
+  - "pct_change handles negative values"
+test_file_path: "cpp/tests/test_kpi_card_math.cpp"
 file_path: "cpp/functions/viz/kpi_card.cpp"
 framework: imgui
 params: