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/conformance.conf

@@ -28,6 +28,7 @@ PRELOADS=(
   lib/search/page.sx
   lib/search/fuzzy.sx
   lib/search/highlight.sx
+  lib/search/stem.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -42,4 +43,5 @@ SUITES=(
   "page:lib/search/tests/page.sx"
   "fuzzy:lib/search/tests/fuzzy.sx"
   "highlight:lib/search/tests/highlight.sx"
+  "stem:lib/search/tests/stem.sx"
 )