Constant RESERVED_TOKEN_FLAG 

pub const RESERVED_TOKEN_FLAG: u64 = _; // 9_223_372_036_854_775_808u64

Top bit marking a u64 as a reserved spawn token rather than a packed NodeHandle. A real Entity::to_bits sets this bit only after a single index is recycled ~2^31 times, which does not happen in a UI session; handles are opaque and round-trip only through the provided helpers, per the wire-format contract.