dlist.nx

dlist.nx -- intrusive doubly-linked list. Each node carries prev/next pointers; list has a sentinel node (heap-allocated) whose prev/next thread the ring. Intrusive: the node pointer is the handle the caller holds, not a wrapper around T. Trade-offs vs std::list-style wrapped list: + O(1) remove given just a node pointer (no search) + one user struct can participate in multiple lists (LRU + pending + free) with separate DNode fields - caller allocates nodes Design note: NishiLang v1 doesn't allow `&struct.field` for inline fields, so DList stores the sentinel as a separately- allocated *DNode rather than an embedded DNode. Small extra alloc at list-construction time only. Used by: - LRU caches (move-to-front, evict tail) - scheduler run queues - free-block chains in allocators - order-preserving map iteration (map + sibling dlist) Invariants: DL1 Empty list: sentinel.next == sentinel.prev == sentinel. DL2 dlist_insert_after + dlist_remove maintain n.next.prev == n for all n. DL3 dlist_remove does NOT free the node -- caller owns lifetime.