Add computed+HO tests, remove duplicate pretext-layout-lines define

- 7 new tests in computed-ho-forms suite: computed with map, reduce,
  for-each, nested map, dict creation, signal updates. All pass on
  OCaml and WASM sandbox.
- Removed standalone pretext-position-line and pretext-layout-lines
  from pretext-demo.sx — now in text-layout library only
- Root cause of island error: pretext-demo.sx had old define with
  (reduce + 0 lwid) that the server serialized into component defs,
  overriding the library's sum-loop version

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

spec/tests/test-unified-reactive.sx

@@ -598,3 +598,87 @@
         (assert-equal
           "hello world"
           (bind (str (context :first) " " (context :second))))))))
+
+(defsuite
+  "computed-ho-forms"
+  (deftest
+    "computed with map"
+    (let
+      ((s (signal 10))
+        (c
+          (computed (fn () (map (fn (x) (* x (deref s))) (list 1 2 3))))))
+      (assert-equal (list 10 20 30) (deref c))))
+  (deftest
+    "computed with map then reduce"
+    (let
+      ((s (signal 10))
+        (c
+          (computed
+            (fn
+              ()
+              (let
+                ((widths (map (fn (x) (* x (deref s))) (list 1 2 3))))
+                (reduce + 0 widths))))))
+      (assert-equal 60 (deref c))))
+  (deftest
+    "computed with map+reduce updates on signal change"
+    (let
+      ((s (signal 10))
+        (c
+          (computed
+            (fn
+              ()
+              (reduce + 0 (map (fn (x) (* x (deref s))) (list 1 2 3)))))))
+      (assert-equal 60 (deref c))
+      (reset! s 5)
+      (assert-equal 30 (deref c))))
+  (deftest
+    "computed with for-each and mutation"
+    (let
+      ((s (signal 2))
+        (c
+          (computed
+            (fn
+              ()
+              (let
+                ((acc (list)))
+                (for-each
+                  (fn (x) (append! acc (* x (deref s))))
+                  (list 10 20 30))
+                acc)))))
+      (assert-equal (list 20 40 60) (deref c))))
+  (deftest
+    "computed with nested map"
+    (let
+      ((s (signal 1))
+        (c
+          (computed
+            (fn
+              ()
+              (map
+                (fn (row) (map (fn (x) (+ x (deref s))) row))
+                (list (list 1 2) (list 3 4)))))))
+      (assert-equal (list (list 2 3) (list 4 5)) (deref c))))
+  (deftest
+    "computed with map producing dicts"
+    (let
+      ((s (signal 10))
+        (c
+          (computed (fn () (map (fn (w) {:width (* (len w) (deref s)) :word w}) (list "hi" "there"))))))
+      (let
+        ((result (deref c)))
+        (assert-equal 2 (len result))
+        (assert-equal "hi" (get (first result) :word))
+        (assert-equal 20 (get (first result) :width)))))
+  (deftest
+    "computed with map+dict then sum widths"
+    (let
+      ((s (signal 10))
+        (c
+          (computed
+            (fn
+              ()
+              (let
+                ((items (map (fn (w) {:width (* (len w) (deref s)) :word w}) (list "hi" "there" "world"))))
+                (reduce + 0 (map (fn (item) (get item :width)) items)))))))
+      (assert-equal 120 (deref c)))))