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>

lib/search/conformance.conf

@@ -27,6 +27,7 @@ PRELOADS=(
   lib/search/prefix.sx
   lib/search/page.sx
   lib/search/fuzzy.sx
+  lib/search/highlight.sx
   lib/search/api.sx
   lib/search/testlib.sx
 )
@@ -40,4 +41,5 @@ SUITES=(
   "prefix:lib/search/tests/prefix.sx"
   "page:lib/search/tests/page.sx"
   "fuzzy:lib/search/tests/fuzzy.sx"
+  "highlight:lib/search/tests/highlight.sx"
 )