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>

spec/tests/test-hyperscript-conformance.sx

@@ -22,7 +22,8 @@
               (list (quote hs-type-check!) hs-type-check!)
               (list (quote hs-matches?) hs-matches?)
               (list (quote hs-coerce) hs-coerce)
-              (list (quote hs-contains?) hs-contains?)))
+              (list (quote hs-contains?) hs-contains?)
+              (list (quote hs-empty?) hs-empty?)))
           (overrides (list)))
         (do
           (when
@@ -51,17 +52,19 @@
               (list (quote let) defaults (list (quote let) overrides sx)))))))))
 
 ;; ── run-hs-fixture: evaluate one test case ────────────────────────────
-(define
-  run-hs-fixture
-  (fn
-    (f)
-    (let
-      ((src (get f "src"))
-        (expected (get f "expected"))
-        (ctx (if (or (get f "locals") (get f "me")) {:me (get f "me") :locals (get f "locals")} nil)))
+(begin
+  (define _hs-error-sentinel "_HS_EVAL_ERROR_")
+  (define
+    run-hs-fixture
+    (fn
+      (f)
       (let
-        ((result (if ctx (eval-hs src ctx) (eval-hs src))))
-        (assert= result expected src)))))
+        ((src (get f "src"))
+          (expected (get f "expected"))
+          (ctx (if (or (get f "locals") (get f "me")) {:me (get f "me") :locals (get f "locals")} nil)))
+        (let
+          ((result (if ctx (eval-hs src ctx) (eval-hs src))))
+          (assert= result expected src))))))
 
 ;; ── arrayIndex (1 fixtures) ──────────────────────────────
 (defsuite