docs(w43): add edgevec-langchain README — Day 6 documentation sprint

Complete package documentation covering installation, quick start, full API
reference, filter DSL syntax (all 27 operators), WASM initialization guide,
IndexedDB persistence, and score normalization. Hostile review APPROVED
(round 2, 0 critical/0 major).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: matte1782

pkg/.gitignore

@@ -1,6 +1,7 @@
 *
 # Allow langchain adapter source (not build output)
 !langchain/
+!langchain/README.md
 !langchain/package.json
 !langchain/tsconfig.json
 !langchain/tsup.config.ts