2026-10-15

Document exec marker preservation

Summary:
Transition authors can return a replacement `PlatformInfo`, but the effective exec configuration also retains the marker owned by Buck2. The existing guides did not explain this exception, leaving it unclear what happens when a transition omits the marker or sets it to `false`.

Document the contract in `ExecutionPlatformRegistrationInfo.exec_marker_constraint` and the execution platform and transition guides: constructors and single/split transitions restore a missing marker, while explicitly returning a different value is an error. Transition authors can omit the marker and let Buck2 preserve it. Manually supplying the same constraint on an ordinary target does not enable restoration or conflict rejection. Clarify the registration's fallback scope and the marker's presence during idempotence validation. Link the conceptual transition guide to this behavior, including its effect on otherwise empty outputs.

Reviewed By: NavidQar

Differential Revision: D124812251

fbshipit-source-id: 778d5905f00f813e64ddc71af9b7844455a5cc7d