0.6.0

Added

• Tree stores keep long text once. A new layout, eventlog-tree/2, may keep a blob that is a JSON
  document (or JSON Lines) as a split blob: a manifest under split-blobs/ whose string literals
  of 1,024 bytes or more are texts under texts/, each stored once per tenant under its SHA-256.
  A literal that spells bytes in hexadecimal keeps those bytes once, and JSON inside them is cut
  the same way. Readers serve exactly the blob's bytes under the same digest and refuse a manifest
  whose expansion has another length or SHA-256. New stores are created as eventlog-tree/2.
  Design: docs/design/tree-text-blobs-v0.1.md.
• eventlog_tree::migrate(root, MigrationMode::DryRun | Apply) moves an eventlog-tree/1 store
  to eventlog-tree/2. It verifies the store first, converts one blob at a time with a read-back
  before the raw file goes, then reloads the store and checks every blob, group and event is served
  as before. It is idempotent and never runs on open. On a copy of a 9,682-blob planning store it
  cut history bytes from 180,651,308 to 77,406,062 and the largest file from 75,332,948 to
  5,153,913 bytes.

Changed

• Every eventlog-tree/1 store still opens, reads and writes exactly as before. It is refused by
  name if it holds split-blob or text files. verify's V2 rule accepts the migration commit: a
  blob file may move between forms when the head serves the same bytes, and store.json may move
  from eventlog-tree/1 to eventlog-tree/2 with the same identity.
• Deleting a blob of an eventlog-tree/2 store removes every text no other blob names.
• New and migrated stores are refused by earlier readers, which accept only
  eventlog-tree/1.