pub const RANGE_BATCHES: u64 = _; // 1_048_576u64Expand description
One permanent range contains this many 128-key batches.
2^20 = 1,048,576 batches, so 2^27 one-time keys per range record. Range
number r always covers batch IDs r × 2^20 up to (not including)
(r + 1) × 2^20. The mapping is fixed, so no “next free batch” counter
has to be updated on disk, and there is no such counter to corrupt.