fix @ root remapping in Blackboard::createEntryImpl

[aysha-afrah26]

A subtree port that remaps to a root blackboard key, written as port="{@foo}" in the XML, is stored as the remapping param -> @foo. When that entry is first created, createEntryImpl walks the remapping up to the parent but never applies the @ redirection that getEntry, set, and createEntry all use, so it lands as a literal @foo entry in the root storage. getEntry then strips the @ and looks up foo in the root, which never matches, so the two disagree about where the entry actually lives. I ran into it through ImportBlackboardFromJSON, the public restore path behind ImportTreeFromJSON: it looks the key up, misses, creates it, looks again, still gets null, and then locks entry_mutex on that null pointer for a segfault. The same split loses data silently as well, since set("param") writes into @foo while a reader gets foo from the root. Moving the @ handling getEntry already has into createEntryImpl keeps creation and lookup in agreement, and the added test crashes before the change and passes after.