QuickJS: make Fetch finalization pool-independent

QuickJS: make Fetch finalization pool-independent

QuickJS may defer finalization of a Fetch object until after its native
backing pools have been destroyed.  Context reuse is not required: a
local JavaScript cycle is sufficient.

    function content(r) {
        const response = new Response("body");

        response.self = response;
        r.return(200);
    }

With context reuse disabled, the cycle is collected while
JS_FreeContext() destroys the per-request context.  At that point the
engine pool has already been destroyed.  The Response finalizer accessed
the pool-backed native structure and attempted to destroy its response
chain, causing a use-after-free.  Request finalization had the same
lifetime dependency through its cached Headers value.

The fix introduces a QuickJS-runtime-owned wrapper for Fetch Request and
Response objects.  It owns the cached Headers value and keeps only a
non-owning pointer to the pool-backed data.  Finalizers now release only
runtime-owned state, while response chains continue to be reclaimed in
bulk with the engine pool.