Returns 1 if the reference graph of `value` contains a loop (a
structure that can reach itself), 0 otherwise.
The walk follows array and class items, mapping keys and values, a
function pointer's captured argument list, and a promise's settled
value plus every pending reaction on it (the fulfillment and rejection
handlers, the chained promise, and -- for a reaction holding a parked
async frame -- that frame's saved value-stack slots, its pending
defer() handlers and its result promise). Objects are leaves: a loop
routed through an object's global variables is reclaimed by destruct()
and is not reported here.
Sharing without a loop (the same array referenced from two places) is
NOT a cycle and returns 0.
The traversal is iterative, so unlike save_variable() or copy() it has
no nesting-depth limit: arbitrarily deep acyclic values scan cleanly.
Values containing a loop cannot be saved, deep-copied, or fully
printed, and once the last outside reference is dropped they leak
permanently (the driver reclaims values by reference counting and has
no cycle collector). See the "Reference Loops" concepts page for the
full story.