fix(psi): make LuaDeclarationTreeStub position map thread-safe

[KSkun]

LuaDeclarationTreeStub.getPosition mutates a plain LinkedHashMap (getOrPut), and stub trees can be built and used by several inspection threads at once (LuaDeclarationTree.get is check-then-set, and scopes are cached on shared PSI). A concurrent put/resize corrupts the map; the surviving thread then loops inside a read action and blocks the write lock until the IDE is killed.

Seen twice in Rider 2026.2.3.1 / EmmyLua 1.4.26. Both freezes had one busy thread, in HashMap$TreeNode.split (HashMap.java:2304), reached from RemoveUnusedLocal -> LuaIndexReference.resolve -> TyClass.findMemberType ->
LuaDeclarationTree.get -> buildTree -> getPosition. The UI thread was waiting for a write-intent permit.

Fix: ConcurrentHashMap.computeIfAbsent + AtomicInteger.