fix(query-db-collection): implement reference counting for QueryObserver lifecycle

.changeset/fix-query-collection-remount-cache.md

@@ -0,0 +1,27 @@
+---
+"@tanstack/query-db-collection": patch
+"@tanstack/db": patch
+---
+
+Fix data loss on component remount by implementing reference counting for QueryObserver lifecycle
+
+**What changed vs main:**
+
+Previously, when live query subscriptions unsubscribed, there was no tracking of which rows were still needed by other active queries. This caused data loss during remounts.
+
+This PR adds reference counting infrastructure to properly manage QueryObserver lifecycle:
+
+1. Pass same predicates to `unloadSubset` that were passed to `loadSubset`
+2. Use them to compute the queryKey (via `generateQueryKeyFromOptions`)
+3. Use existing machinery (`queryToRows` map) to find rows that query loaded
+4. Decrement the ref count
+5. GC rows where count reaches 0 (no longer referenced by any active query)
+
+**Impact:**
+
+- Navigation back to previously loaded pages shows cached data immediately
+- No unnecessary refetches during quick remounts (< gcTime)
+- Multiple live queries with identical predicates correctly share QueryObservers
+- Proper row-level cleanup when last subscriber leaves
+- TanStack Query's cache lifecycle (gcTime) is fully respected
+- No data leakage from in-flight requests when unsubscribing