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-compiler.sx

@@ -115,7 +115,7 @@
     "log passes through"
     (let
       ((sx (hs-to-sx-from-source "log 'hello'")))
-      (assert= (quote log) (first sx))
+      (assert= (quote console-log) (first sx))
       (assert= "hello" (nth sx 1))))
   (deftest
     "append becomes dom-append"