v2.18.2 — kg_TXT fix, iFind fallback, fusion silent-degrade

v2.18.2 (2026-09-07)

Fix: kg_TXT SQLCODE -51 / silent hybrid-search degradation

• kg_TXT SQLCODE -51: CALL iris_vector_graph.kg_TXT(...) failed because IRIS
  registers the stored procedure as a scalar FUNCTION. Rewrote to inline SQL using
  %FIND/%FIND.Rank when the iFind index exists, with a LIKE-based fallback on
  instances without iFind installed.
• docs.text iFind index: Added idx_docs_text_ifind to the base DDL and to
  ensure_indexes so kg_TXT gets BM25-ranked results on fresh installs.
• kg_TXT DDL arity: Schema DDL had 2 params (q, k); Python caller passed 3
  (query_text, k, min_confidence). DDL updated to 3 params; kg_RRF_FUSE call site
  updated to kg_TXT(:qtext, :k2, 0).
• Silent hybrid degradation fixed: fusion.py:156 swallowed kg_TXT exceptions as
  a WARNING, making hybrid search silently degrade to vector-only with no caller-visible
  error. Changed to logger.error + re-raise.
• E2E regression tests: tests/integration/test_kg_txt_e2e.py — 6 tests covering
  result shape, score types, k limit, no-match, CALL-verb detection, and fusion
  exception propagation.