Playwright sandbox: offline browser test environment for WASM kernel

New sx_playwright mode="sandbox" — injects the WASM kernel into about:blank
with full FFI, IO suspension tracing, and real DOM. No server needed.

Predefined stacks: core (kernel only), web (full web stack), hs (+ hyperscript),
test (+ test framework). Custom files and setup expressions supported.

Reproduces the host-callback IO suspension bug: direct callFn chains 6/6
suspensions correctly, but host-callback → addEventListener → _driveAsync
only completes 1/6. Bug is in the _driveAsync resume chain context.

Also: debug.sx mock DOM harness, test_hs_repeat.js Node.js reproduction.

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

lib/hyperscript/runtime.sx

@@ -295,7 +295,15 @@
 
 (define
   hs-falsy?
-  (fn (v) (or (nil? v) (= v false) (and (string? v) (= v "")))))
+  (fn
+    (v)
+    (cond
+      ((nil? v) true)
+      ((= v false) true)
+      ((and (string? v) (= v "")) true)
+      ((and (list? v) (= (len v) 0)) true)
+      ((= v 0) true)
+      (true false))))
 
 (define
   hs-matches?
@@ -313,4 +321,15 @@
     (cond
       ((list? collection) (some (fn (x) (= x item)) collection))
       ((string? collection) (string-contains? collection item))
+      (true false))))
+
+(define
+  hs-empty?
+  (fn
+    (v)
+    (cond
+      ((nil? v) true)
+      ((string? v) (= (len v) 0))
+      ((list? v) (= (len v) 0))
+      ((dict? v) (= (len (keys v)) 0))
       (true false))))