Fix(rtl-generics): clear FQueue.FIdx in TDeamortizedDArrayCuckooMap.Clear

After Clear, FQueue.FItems was set to nil but FQueue.FIdx (the index list tracking queue item positions) was left with stale entries. The next Add call would InsertIntoBack a new item into the freshly allocated FItems, then Pop would dequeue a stale index and pass it to DoRemove, causing an out-of-bounds access and SIGSEGV.

It was reported in fpc-devel mailing list: https://lists.freepascal.org/pipermail/fpc-devel/2026-April/046332.html