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>

lib/search/tests/stem.sx

@@ -0,0 +1,47 @@
+;; Extension — stemming (suffix stripping). Scalar string results wrapped in [].
+
+(define
+  stem-cases
+  (list
+    (list "stem plural s" "[stem \"cats\"]" (list "cat"))
+    (list "stem plural dogs" "[stem \"dogs\"]" (list "dog"))
+    (list "stem keeps ss" "[stem \"pass\"]" (list "pass"))
+    (list "stem short s unchanged" "[stem \"is\"]" (list "is"))
+    (list "stem es boxes" "[stem \"boxes\"]" (list "box"))
+    (list "stem es wishes" "[stem \"wishes\"]" (list "wish"))
+    (list "stem ies cities" "[stem \"cities\"]" (list "city"))
+    (list "stem ies parties" "[stem \"parties\"]" (list "party"))
+    (list "stem ing jumping" "[stem \"jumping\"]" (list "jump"))
+    (list "stem ing running literal" "[stem \"running\"]" (list "runn"))
+    (list "stem ed jumped" "[stem \"jumped\"]" (list "jump"))
+    (list "stem ed wanted" "[stem \"wanted\"]" (list "want"))
+    (list "stem short ed unchanged" "[stem \"red\"]" (list "red"))
+    (list "stem no suffix" "[stem \"cat\"]" (list "cat"))
+    (list
+      "stemText normalizes and stems"
+      "[stemText \"Cats Running!\"]"
+      (list "cat runn"))
+    (list
+      "stemTokens list"
+      "stemTokens \"boxes and cats\""
+      (list "box" "and" "cat"))
+    (list
+      "indexStemmed unifies plural"
+      "map fst (lookupTerm \"cat\" (indexStemmed 2 \"a cat\" (indexStemmed 1 \"the cats\" emptyIndex)))"
+      (list 1 2))
+    (list
+      "indexStemmed stem query"
+      "map fst (lookupTerm (stem \"boxes\") (indexStemmed 1 \"many boxes\" emptyIndex))"
+      (list 1))))
+
+(define
+  stem-results
+  (search-batch "" (map (fn (c) (nth c 1)) stem-cases)))
+
+(map-indexed
+  (fn
+    (i c)
+    (hk-test (nth c 0) (nth stem-results i) (nth c 2)))
+  stem-cases)
+
+{:fail hk-test-fail :pass hk-test-pass :fails hk-test-fails}