Add versioned startup backfill state so the expensive summary and tool-call
repairs only run once per algorithm version. Keep retry safety by wrapping each
versioned backfill and its completion marker in a savepoint so a failed startup
rolls back partial backfill writes and reruns cleanly on the next launch.

Regeneration-Prompt: |
  Implement the startup backfill gating change in lossless-claw without using
  PRAGMA user_version or column-existence guesses as the completion signal.
  Add an additive SQLite table keyed by backfill step name and algorithm
  version, and only skip a backfill after that exact version completes.
  Preserve partial-upgrade safety by making the backfill work and state write
  succeed or roll back together, then cover first-run state creation, repeat
  startup skipping, and retry-after-failure behavior in migration tests. This
  runtime change affects package behavior, so include a patch changeset.