pub const RANGE_EPOCH_BIT: u64 = _; // 9_223_372_036_854_775_808u64Expand description
Epoch high bit reserved for fixed-range allocation; single leases reject it.
Bit 63 of the 64-bit epoch splits the epoch space into two halves: epochs
with this bit clear belong to BatchLease::reserve, epochs with it set
belong to RangeLease::reserve. The split exists so a single-batch
lease and a range lease can never name the same batch, even though their
record files have different names and so would not collide on disk.