search: stemming (suffix stripping) + 18 tests

Deterministic English suffix stripping (stem), stemText/stemTokens, indexStemmed.
Worked around two haskell-on-sx string gotchas: take/drop over a String yield
char codes (rebuild via joinChars . map chr), and isSuffixOf's reverse trips ++
(manual suffix compare). 196/196.

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

plans/search-on-sx.md

@@ -116,10 +116,16 @@ lib/search/index.sx                     lib/search/eval.sx
 - [x] result pagination (offset / limit) — `paginate`, `pageTfIdf`, `pageBm25`,
   `resultCount` — 12 tests
 - [x] snippet / highlight generation (`highlight`, `snippet`) — 12 tests
-- [ ] stemming (suffix stripping) — recall-improving normalizer
+- [x] stemming (suffix stripping) — `stem`, `stemText`, `stemTokens`, `indexStemmed`
+  — 18 tests
 
 ## Progress log
 
+- **Extension: stemming (196/196 total).** Deterministic English suffix stripping
+  (`stem`), `stemText`/`stemTokens`, `indexStemmed`. Two haskell-on-sx gotchas: take/drop
+  over a String yield char CODES not char strings (rebuild via `joinChars . map chr`),
+  and isSuffixOf's `reverse` trips `++` on the String repr (manual suffix compare). All
+  five planned extensions now done; the loop can keep adding search vocabulary. 18 tests.
 - **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.