Make SubscriptionRegistry.TryResolveItemHandle confirm a live subscription
genuinely binds fullRef->handle (via the reverse index) rather than trusting
the forward-map hint + a bare liveness check. Fixes the cross-ref-same-handle
hazard (wrong-tag borrow) while preserving the legitimate
multiple-subscriptions-per-tag borrow. Adds cross-ref + same-ref-multi-sub
tests; drops a duplicate SubscriptionEntry <summary>; documents the writer's
supervisory-advise reconnect lifecycle.
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