Restore hyperscript work on stable site base (908f4f80)

Reset to last known-good state (908f4f80) where links, stepper, and
islands all work, then recovered all hyperscript implementation,
conformance tests, behavioral tests, Playwright specs, site sandbox,
IO-aware server loading, and upstream test suite from f271c88a.

Excludes runtime changes (VM resolve hook, VmSuspended browser handler,
sx_ref.ml guard recovery) that need careful re-integration.

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

spec/tests/test-hyperscript-parser.sx

@@ -205,7 +205,8 @@
       (assert=
         (list
           (quote increment!)
-          (list (quote attr) "count" (list (quote me))))
+          (list (quote attr) "count" (list (quote me)))
+          (list (quote me)))
         ast)))
   (deftest
     "decrement attribute"
@@ -214,7 +215,8 @@
       (assert=
         (list
           (quote decrement!)
-          (list (quote attr) "score" (list (quote me))))
+          (list (quote attr) "score" (list (quote me)))
+          (list (quote me)))
         ast)))
   (deftest
     "hide"
@@ -754,7 +756,8 @@
       (assert=
         (list
           (quote increment!)
-          (list (quote attr) "count" (list (quote me))))
+          (list (quote attr) "count" (list (quote me)))
+          (list (quote me)))
         ast)))
   (deftest
     "on click from #bar add .clicked → full AST"