A node in the mutable doubly-linked list of WriterEvents. We use [] instead of option for Prev/Next links because this is a hot path — every formatting operation appends nodes. Option would allocate a Some wrapper on every link assignment, adding GC pressure for no functional benefit. The null checks are contained within EventList's methods; callers work with non-null EventNode references returned by Append/InsertAfter/InsertBefore.