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/compiler.sx

@@ -333,7 +333,7 @@
                   (hs-to-sx (nth ast 1))
                   (hs-to-sx (nth ast 2))))
               ((= head (quote empty?))
-                (list (quote empty?) (hs-to-sx (nth ast 1))))
+                (list (quote hs-empty?) (hs-to-sx (nth ast 1))))
               ((= head (quote exists?))
                 (list
                   (quote not)
@@ -356,10 +356,13 @@
                   (hs-to-sx (nth ast 2))
                   (hs-to-sx (nth ast 1))))
               ((= head (quote of))
-                (list
-                  (quote get)
-                  (hs-to-sx (nth ast 2))
-                  (hs-to-sx (nth ast 1))))
+                (let
+                  ((prop (hs-to-sx (nth ast 1)))
+                    (target (hs-to-sx (nth ast 2))))
+                  (cond
+                    ((= prop (quote first)) (list (quote first) target))
+                    ((= prop (quote last)) (list (quote last) target))
+                    (true (list (quote get) target prop)))))
               ((= head "!=")
                 (list
                   (quote not)