search: prefix/wildcard queries + 14 tests

prefixTerms matches indexed terms by prefix (allTerms + isPrefixOf); prefixDocs
unions their docs; prefixRankTfIdf ranks via the matched terms. 136/136.

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

lib/search/tests/prefix.sx

@@ -0,0 +1,63 @@
+;; Extension — prefix / wildcard queries.
+;; Corpus: 1 "alpha alpine"  2 "beta apple"  3 "banana alpha"
+;; allTerms sorted: alpha alpine apple banana beta
+
+(define
+  prefix-setup
+  "idx = indexDoc 3 \"banana alpha\" (indexDoc 2 \"beta apple\" (indexDoc 1 \"alpha alpine\" emptyIndex))\n")
+
+(define
+  prefix-cases
+  (list
+    (list
+      "prefix terms two matches"
+      "prefixTerms \"al\" idx"
+      (list "alpha" "alpine"))
+    (list
+      "prefix terms narrower"
+      "prefixTerms \"alp\" idx"
+      (list "alpha" "alpine"))
+    (list
+      "prefix terms wide"
+      "prefixTerms \"a\" idx"
+      (list "alpha" "alpine" "apple"))
+    (list "prefix terms single" "prefixTerms \"ban\" idx" (list "banana"))
+    (list "prefix terms exact term" "prefixTerms \"beta\" idx" (list "beta"))
+    (list "prefix terms none" "prefixTerms \"z\" idx" (list))
+    (list
+      "prefix docs union"
+      "prefixDocs \"al\" idx"
+      (list 1 3))
+    (list "prefix docs single term" "prefixDocs \"ban\" idx" (list 3))
+    (list
+      "prefix docs wide"
+      "prefixDocs \"a\" idx"
+      (list 1 2 3))
+    (list "prefix docs none" "prefixDocs \"z\" idx" (list))
+    (list
+      "prefix docs exact"
+      "prefixDocs \"alpha\" idx"
+      (list 1 3))
+    (list
+      "prefix rank ranks by matched terms"
+      "prefixRankTfIdf \"al\" idx"
+      (list 1 3))
+    (list
+      "prefix rank single doc"
+      "prefixRankTfIdf \"ban\" idx"
+      (list 3))
+    (list "prefix rank empty" "prefixRankTfIdf \"z\" idx" (list))))
+
+(define
+  prefix-results
+  (search-batch
+    prefix-setup
+    (map (fn (c) (nth c 1)) prefix-cases)))
+
+(map-indexed
+  (fn
+    (i c)
+    (hk-test (nth c 0) (nth prefix-results i) (nth c 2)))
+  prefix-cases)
+
+{:fail hk-test-fail :pass hk-test-pass :fails hk-test-fails}