test: add 265 unit tests across 6 critical Rust crates

VeriSimDB had zero unit tests despite being the backing store for the
entire ecosystem. This adds meaningful tests with real assertions to
the 6 most critical crates:

- verisim-graph (19 tests): node creation, edge CRUD, deduplication,
  neighborhood traversal, literal vs node indexing, error display
- verisim-vector (31 tests): embedding ops, cosine/euclidean/dot-product
  similarity, dimension mismatch errors, upsert/delete, normalization
- verisim-drift (65 tests): severity classification, threshold policies
  (fixed + adaptive), moving average convergence, event channels,
  calculator algorithms for all 8 drift types, TensorStats with NaN/Inf
- verisim-octad (52 tests): OctadId generation/equality/hashing/serialization,
  ModalityStatus completeness checks, OctadBuilder all-modality construction,
  OctadConfig defaults, error display
- verisim-document (17 tests): index/search/get/delete lifecycle, snippet
  generation, search limits, document replacement, multi-doc queries
- verisim-api (81 tests): validation functions (ID, vector, limit), error
  response codes, request-to-input conversion, CRUD endpoints, vector
  search, drift/normalizer/metrics status, Groove well-known manifest

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath