Add 21 new tests: scope/context, multi-signal, reactive DOM, VM HO forms

WASM browser tests (web/tests/test-wasm-browser.sx): 32 tests
- scope/context: push+read, default, nesting, pop reveals outer
- multi-signal: effect tracks 2 signals, computed from 2 sources,
  unsubscribe on dependency change
- reactive DOM: computed expression attrs, map rendering, component
  rendering, spread signal update, style attributes
- conditional: if/when/cond/let/begin in render-to-dom

VM HO form tests (lib/tests/test-vm.sx): +8 tests
- map, filter, reduce, for-each, some, every? in vm-eval
- nested map, map with closure over local

32/32 WASM (source+bytecode), 1593/1593 JS full (1 pre-existing silent).

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

lib/tests/test-vm.sx

@@ -730,3 +730,51 @@
         (quote (define run (fn () (+ (inc 0) (inc 10) (inc 20)))))
         g)
       (assert-equal 33 (vm-eval-with (quote (run)) g)))))
+
+(defsuite
+  "vm-ho-forms"
+  (deftest
+    "map in vm-eval"
+    (assert-equal
+      (list 2 4 6)
+      (vm-eval (quote (map (fn (x) (* x 2)) (list 1 2 3))))))
+  (deftest
+    "filter in vm-eval"
+    (assert-equal
+      (list 2 3 4)
+      (vm-eval (quote (filter (fn (x) (> x 1)) (list 1 2 3 4))))))
+  (deftest
+    "reduce in vm-eval"
+    (assert-equal
+      10
+      (vm-eval (quote (reduce (fn (acc x) (+ acc x)) 0 (list 1 2 3 4))))))
+  (deftest
+    "for-each with set! in vm-eval"
+    (assert-equal
+      6
+      (vm-eval
+        (quote
+          (let
+            ((sum 0))
+            (for-each (fn (x) (set! sum (+ sum x))) (list 1 2 3))
+            sum)))))
+  (deftest
+    "some in vm-eval"
+    (assert-true
+      (vm-eval (quote (some (fn (x) (> x 3)) (list 1 2 3 4))))))
+  (deftest
+    "every? in vm-eval"
+    (assert-false
+      (vm-eval (quote (every? (fn (x) (> x 0)) (list 1 2 -1))))))
+  (deftest
+    "nested map in vm-eval"
+    (assert-equal
+      (list 1 4 9)
+      (vm-eval (quote (map (fn (x) (* x x)) (list 1 2 3))))))
+  (deftest
+    "map with closure over local"
+    (assert-equal
+      (list 11 12 13)
+      (vm-eval
+        (quote
+          (let ((offset 10)) (map (fn (x) (+ x offset)) (list 1 2 3))))))))