ADR-099: Int64/Uint64 as first-class kinds. The POC targets (SFTP
offset: u64, metatensor data_offsets: u64) require 64-bit integers.
The prior Uint64 addition was removed because it was half-finished;
this ADR specifies the complete addition (layout + validator + API).
ADR-100: Reject non-final inline length-prefixed variable fields in
aligned mode. The OffsetMap reserves only 4 bytes (the length prefix),
but write_string writes prefix+data inline — clobbering subsequent
fields. Non-final variable fields must use maxLength or offset-indirect.
ADR-101: Packed-mode read API — engine as SequentialReader factory.
engine.sequential_reader() returned &SequentialReader but read_next
needs &mut self — dead API. Now returns an owned fresh reader.
ADR-102: Reject TUnion in aligned mode for v1. The aligned-mode union
code had three bugs (no variant offsets, first-variant discriminator
offset, misaligned variant region). Unions are the protocol pattern;
mmap formats use structs/arrays. Reversible when a consumer needs it.
