track wrapped iterator removability in ListIteratorWrapper

[rootvector2]

ListIteratorWrapper.remove() gates on a removeState flag that next() sets only in the branch that pulls a fresh element from the wrapped iterator, and previous() recomputes it from the cursor position alone, so walking back into the element cache with previous() and then forward again until the cursor is level with the wrapped iterator leaves the flag false and remove() throws IllegalStateException for a removal java.util.ListIterator requires to succeed, while reset() rewinds the cursor without clearing the flag so next(), reset(), remove() deletes an element the remove() javadoc says it must reject; the flag is now derived from whether the element last returned by the wrapped iterator can still be removed from it. Found with a randomized differential test of the library's list iterators against java.util.ArrayList.listIterator(), which also confirmed ListIteratorWrapper is the only one that caches a plain Iterator this way (the siblings either delegate to a real ListIterator or reject remove() outright).

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