fix: refactor GC gray set to use count/capacity and handle empty set edge cases

- Rename grayDepth to grayCount and maxGray to grayCapacity for clarity
- Fix gray stack growth condition to check capacity instead of depth
- Use config.reallocateFn directly instead of wrenReallocate wrapper
- Change blacken loop from do-while to while to handle empty gray set
- Remove separate initializeGC function and inline allocation in wrenNewVM
- Add explicit System.gc() calls in deeply_nested_gc and many_reallocations tests
- Normalize test output strings to lowercase "done"

test/language/deeply_nested_gc.wren

@@ -4,4 +4,5 @@ for (i in 1..400000) {
   head = { "next" : head }
 }
 
-System.print("done") // expect: done
+System.gc()
+System.print("done") // expect: done

test/language/many_reallocations.wren

@@ -10,5 +10,7 @@ for (i in 1..1000) {
   }
   found.add(bar)
 }
+
+System.gc()
 System.print(found.all {|i| i == 1337}) // expect: true
-System.print("DONE!") // expect: DONE!
+System.print("done") // expect: done