v0.4.0

Added

• retriever::Retriever trait plus VectorStoreRetriever adapter and the
  score_retriever / retrieve_all drivers, so backends that are not vector
  stores (lexical / BM25 / hybrid rerankers, remote search APIs) can be scored
  with the same IR metrics. RetrievalHarness now delegates to this driver.
• Qrels::from_beir loads a downloaded BEIR / BRIGHT dataset
  (queries.jsonl + qrels/<split>.tsv, both 3- and 4-column layouts).
• synthetic module: deterministic, seeded "needle in a haystack" corpus +
  qrels generator for reproducible benchmarks and fixture-free tests.
• RetrievalHarness::with_bootstrap and MultiReport::with_bootstrap
  attach percentile-bootstrap confidence intervals to every metric;
  MultiReport::delta_markdown renders a head-to-head delta table.