Development record

The brief at .abcd/development/brief/04-surfaces/21-update.md:48 says: The verb takes an optional release tag; with none, it resolves the latest (no further condition on the bare form). The shipped surface differs: A bare update whose latest release is older than ~/.abcd.noindex/release-mark is refused with shape `older-than-mark` (internal/surface/cli/update.go:107); the update writes release-mark and naming an older tag lowers it (update --help). The chapter documents neither the release-mark record nor this refusal. (v0.14.2 release crosscheck, undocumented-surface, raised by opus in a16.)