[St4NNi]

Description

Stores written by Craqle 0.2 hold version 1 topic cursors. Version 2 added a branch fence, and every reader rejected a version 1 cursor as expired. Reconciliation runs at open, so any node that ever applied a replicated record failed to open once Irokle sync was configured. The audited cursor repair could not help, because it needs an open node with sync configured.

A verified version 1 cursor now counts as no cursor: the topic replays from the start of its current branch, and the first consumed record stores a fenced version 2 cursor. Replay is safe because a record whose dot the graph clock already covers applies as a duplicate. Corrupt cursors still fail and are never reset.

Changes

• Read stored cursors through one helper that maps a verified version 1 cursor to no cursor, used by topic catch-up and by the applied clock of history checks.
• Add legacy_cursor_replays: a replica with a version 1 cursor reconciles, stores a fenced cursor at the same position and exports the same graph.
• Share the version 1 cursor encoding between the decoder test and the new test.
• Note the replay in the 0.2 upgrade section of the changelog.