eb-staged-rebuild 0.4.0

• eb_staged_rebuild.build_events records started, finished and
  aborted ledger events around one build command through a durable
  outbox, and eb_staged_rebuild.ledger_maintenance backfills observed
  history, publishes daily signed anchors, proves the anchor chain and
  projects package history from a verified log (docs/build-events.md,
  docs/ledger-maintenance.md).
• The ledger adapter and the maintenance verifier read the eb-stack 0.5.2
  CLI: the receipt is the first line ledger append, ledger head and
  ledger verify print, exactly one tile-log or checkpoint line for the
  same size and root follows it (none for the size-0 head of an empty
  log), and a tile log is written beside every private ledger copy before
  ledger verify audits it.
• _bounded runs the ledger CLI through pipes and kills its process group
  on every exit, including a signal to the caller, so an interrupted
  delivery leaves no eb-stack process behind.