Add _itemHandleByFullRef (OrdinalIgnoreCase ConcurrentDictionary) maintained
in lock-step with _subscribersByItemHandle across Register/Remove/Rebind.
TryResolveItemHandle cross-checks the authoritative reverse map so a stale
forward entry can never hand out a dead handle. Also wires the scaffolded
_addItemCallCount increment in EnsureItemHandleAsync (field was declared but
never assigned, causing a TreatWarningsAsErrors build failure on the branch).
8 new xUnit + Shouldly facts covering register/case-insensitive/remove/rebind/
failed-handle/liveness-guard paths.
Joseph Doherty