Redesign the final STEP phase to build a step_finali folder

Previously it only compared the top-level BOM against step/ and reported
matches, without copying anything. Now it unions the BOM codes with the
sub-components resolved in phase 4, copies every matching .stp into a new
step_finali/ folder, and reports what got created vs what's still missing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -28,6 +28,7 @@ publish/
 /*.csv
 /raw/
 /step/
+/step_finali/
 *.dxc
 
 # Report BOM generati manualmente da Creo per debug/validazione: mai una dipendenza