Breaking changes for v2

Megathread for breaking changes. This may be a ways off, and the list will almost certainly grow.

- [ ] change GTM to return an unsigned value in [0, 2^24) rather than a signed value in [0, 2^23)

Start by locating the GTM implementation and any tests covering its return range. Confirm the current signed [0, 2^23) behavior and the expected unsigned [0, 2^24) behavior before making the v2 change. Done means the new behavior is implemented, tested, and reflected in this breaking-changes item.

Written by the indexing model from the issue text.