search: highlight + snippet generation + 12 tests

highlight marks query-matching (normalized) tokens with [..]; snippet extracts a
context window around the first match. 178/178.

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

plans/search-on-sx.md

@@ -115,11 +115,14 @@ lib/search/index.sx                     lib/search/eval.sx
   `fuzzyDocs`, `fuzzyRankTfIdf`) — 18 tests
 - [x] result pagination (offset / limit) — `paginate`, `pageTfIdf`, `pageBm25`,
   `resultCount` — 12 tests
-- [ ] snippet / highlight generation
+- [x] snippet / highlight generation (`highlight`, `snippet`) — 12 tests
 - [ ] stemming (suffix stripping) — recall-improving normalizer
 
 ## Progress log
 
+- **Extension: highlight/snippet (178/178 total).** `highlight terms text` marks
+  query-matching (normalized) tokens with [..]; `snippet ctx terms text` extracts a
+  context window around the first match. 12 tests.
 - **Extension: fuzzy matching (166/166 total).** Levenshtein `editDist` as an O(m*n)
   row-based DP (the naive recursive version is exponential and times out under load),
   `fuzzyTerms`/`fuzzyDocs`/`fuzzyRankTfIdf` expand a term to indexed terms within a max