search: result pagination (offset/limit) + 12 tests

paginate windows a ranked list (take lim . drop off); pageTfIdf/pageBm25 and
resultCount. 148/148.

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

lib/search/tests/page.sx

@@ -0,0 +1,53 @@
+;; Extension — result pagination (offset / limit) over ranked results.
+;; Corpus (tf of "x" descending): 1 x4  2 x3  3 x2  4 x1  5 y(no x)
+;; rankTfIdf ["x"] -> [1,2,3,4]
+
+(define
+  page-setup
+  "idx = indexDoc 5 \"y\" (indexDoc 4 \"x\" (indexDoc 3 \"x x\" (indexDoc 2 \"x x x\" (indexDoc 1 \"x x x x other\" emptyIndex))))\n")
+
+(define
+  page-cases
+  (list
+    (list "first page" "pageTfIdf 0 2 [\"x\"] idx" (list 1 2))
+    (list
+      "second page"
+      "pageTfIdf 2 2 [\"x\"] idx"
+      (list 3 4))
+    (list
+      "sliding window"
+      "pageTfIdf 1 2 [\"x\"] idx"
+      (list 2 3))
+    (list
+      "limit exceeds remaining"
+      "pageTfIdf 3 10 [\"x\"] idx"
+      (list 4))
+    (list "offset past end" "pageTfIdf 4 2 [\"x\"] idx" (list))
+    (list "limit zero" "pageTfIdf 0 0 [\"x\"] idx" (list))
+    (list
+      "whole result"
+      "pageTfIdf 0 10 [\"x\"] idx"
+      (list 1 2 3 4))
+    (list
+      "paginate raw list"
+      "paginate 1 2 [10, 20, 30, 40]"
+      (list 20 30))
+    (list "paginate raw past end" "paginate 9 2 [10, 20]" (list))
+    (list
+      "bm25 page window size"
+      "[length (pageBm25 0 2 1.5 0.75 [\"x\"] idx)]"
+      (list 2))
+    (list "result count" "[resultCount [\"x\"] idx]" (list 4))
+    (list "result count zero" "[resultCount [\"zzz\"] idx]" (list 0))))
+
+(define
+  page-results
+  (search-batch page-setup (map (fn (c) (nth c 1)) page-cases)))
+
+(map-indexed
+  (fn
+    (i c)
+    (hk-test (nth c 0) (nth page-results i) (nth c 2)))
+  page-cases)
+
+{:fail hk-test-fail :pass hk-test-pass :fails hk-test-fails}