LeanDB 0.5.1

• Schemas of more than fifteen entities (#139). schema%, and
  native_schema% through it, generated IsSchema.pack as a match on
  Fin literals, which Lean's match compiler rejected with "Missing
  cases" from about the sixteenth entity on. pack now indexes an array
  of the packed entities, so a schema of any size elaborates. The model
  suite declares a twenty-entity schema and writes and reads its first and
  last tables on SQLite.

Lean 4.33.0, leansqlite, and leanontology are unchanged.

Validation: ./scripts/release_check.sh passed, including the engine, DDD and model suites, precompiled consumers, examples, dashboard, scaffolding/import, and HTTP/MCP/host checks; the DDD negative fixtures were rejected as expected. Linux and macOS CI passed.