search: prefix/wildcard queries + 14 tests

prefixTerms matches indexed terms by prefix (allTerms + isPrefixOf); prefixDocs
unions their docs; prefixRankTfIdf ranks via the matched terms. 136/136.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/search-on-sx.md

@@ -108,8 +108,19 @@ lib/search/index.sx                     lib/search/eval.sx
 - [x] `lib/search/tests/integration.sx` — 21 cases: index merge, cross-peer df/lookup,
   position preservation, boolean/phrase over the merge, ACL filter + top-N + bm25
 
+## Extensions (post-roadmap, search-shaped vocabulary)
+
+- [x] prefix / wildcard queries (`prefixTerms`, `prefixDocs`, `prefixRankTfIdf`) — 14 tests
+- [ ] fuzzy matching — edit distance term expansion
+- [ ] result pagination (offset / limit)
+- [ ] snippet / highlight generation
+- [ ] stemming (suffix stripping) — recall-improving normalizer
+
 ## Progress log
 
+- **Extension: prefix/wildcard queries (136/136 total).** `prefixTerms` matches every
+  indexed term starting with a prefix (via allTerms + isPrefixOf); `prefixDocs` unions
+  their docs; `prefixRankTfIdf` ranks treating the matched terms as the query. 14 tests.
 - **Phase 4 complete — federation + ACL (122/122 total). Roadmap done.** `fedIndex`
   merges per-peer inverted indices (union posting lists per term) after relabelling
   local DocIds to global `gid = peer*1000 + local` — the bijection makes (peer,doc-id)