#1263 Looking up navigable node in AST incorrect.

Sometimes (as seen in Ctrl+click in Helios), the search of navigable node at a point is not working correctly. The actual implementation is in https://lolg.it/amber/amber/src/master/lang/src/Compiler-AST.st#L18. The problem is probably the end of method, selection of "the one is closest proximity" from among the correct children should probably be done differently.