Revert the isFull() change: max_slaves counts slots, not contacts

The earlier commit changed MasterElement::isFull() to compare the
contacts in use against max_slaves. That was wrong, and this restores
the original comparison against the number of linked elements.

max_slaves is a number of slots, not of contacts:

 - it sizes the contact group table in the element editor, one row per
   slot (ElementPropertiesEditorWidget::populateSlaveGroupsTable)
 - a group must match the slave's own contact count before it can be
   chosen, so a 4 pole slave needs a group declaring 4 and occupies
   that single group (ContactGroupSelectionDialog)
 - each slave stores exactly one group index
   (Element::setGroupIndexForElement)

So a coil declaring 4 slots accepts 4 slaves, whatever their pole
count. Counting contacts made one 4 pole slave fill a 4 slot coil on
its own and refuse three further links that should have been allowed.

ContactUsage stays, and its per-type tally is still what the General
tab needs: how many contacts an auxiliary block must provide is a
different question from how many slots are occupied, and only the
former wants the pole multiplier. The header now says so.

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

sources/contactusage.h

@@ -29,6 +29,12 @@
 	rules can be unit tested on their own. MasterElement::contactUsage()
 	is the thin wrapper that feeds it the linked elements.
 
+	This counts contacts, which is what tells you how many contacts an
+	auxiliary block must provide. It is deliberately not the count that
+	MasterElement::isFull() uses: a master's max_slaves is a number of
+	slots, and a slave fills exactly one slot however many contacts it
+	carries.
+
 	Two rules are easy to get wrong, and both live here so that every
 	caller gets them right:
 	 - a slave stands for as many contacts as its "number" kind

sources/qetgraphicsitem/masterelement.cpp

@@ -282,8 +282,11 @@ bool MasterElement::isFull() const
 		return false;
 	}
 
-	// Return true if the contacts already used reached or exceeded the limit
-	return contactUsage().total() >= max_slaves;
+		// max_slaves is a number of slots, not of contacts: it sizes the
+		// element's contact group table, and a slave occupies exactly one
+		// group however many contacts that group stands for. So the slots
+		// in use are the linked elements, not the contacts they carry.
+	return connected_elements.size() >= max_slaves;
 }
 
 /**