Whether the fiber is still part of its tree: reachable from its
root's current generation, by identity or alternate. Walking up
alone is unreliable — a deleted fiber's stale return chain can still
reach the root. Hidden (Suspense/Activity) subtrees stay linked, so
they count as live. O(tree size); reserve for cold paths.
Whether the fiber is still part of its tree: reachable from its root's current generation, by identity or alternate. Walking up alone is unreliable — a deleted fiber's stale return chain can still reach the root. Hidden (Suspense/Activity) subtrees stay linked, so they count as live. O(tree size); reserve for cold paths.