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>

lib/hyperscript/integration.sx

@@ -10,6 +10,26 @@
 ;; Returns a function (fn (me) ...) that can be called with a DOM element.
 ;; Uses eval-expr-cek to turn the SX data structure into a live closure.
 
+(load-library! "hs-tokenizer")
+
+;; ── Activate a single element ───────────────────────────────────
+;; Reads the _="..." attribute, compiles, and executes with me=element.
+;; Marks the element to avoid double-activation.
+
+(load-library! "hs-parser")
+
+;; ── Boot: scan entire document ──────────────────────────────────
+;; Called once at page load. Finds all elements with _ attribute,
+;; compiles their hyperscript, and activates them.
+
+(load-library! "hs-compiler")
+
+;; ── Boot subtree: for dynamic content ───────────────────────────
+;; Called after HTMX swaps or dynamic DOM insertion.
+;; Only activates elements within the given root.
+
+(load-library! "hs-runtime")
+
 (define
   hs-handler
   (fn
@@ -25,10 +45,6 @@
             (list (list (quote it) nil) (list (quote event) nil))
             sx))))))
 
-;; ── Activate a single element ───────────────────────────────────
-;; Reads the _="..." attribute, compiles, and executes with me=element.
-;; Marks the element to avoid double-activation.
-
 (define
   hs-activate!
   (fn
@@ -40,22 +56,14 @@
         (dom-set-data el "hs-active" true)
         (let ((handler (hs-handler src))) (handler el))))))
 
-;; ── Boot: scan entire document ──────────────────────────────────
-;; Called once at page load. Finds all elements with _ attribute,
-;; compiles their hyperscript, and activates them.
-
 (define
   hs-boot!
   (fn
     ()
     (let
-      ((elements (dom-query-all (dom-body) "[_]")))
+      ((elements (dom-query-all (host-get (host-global "document") "body") "[_]")))
       (for-each (fn (el) (hs-activate! el)) elements))))
 
-;; ── Boot subtree: for dynamic content ───────────────────────────
-;; Called after HTMX swaps or dynamic DOM insertion.
-;; Only activates elements within the given root.
-
 (define
   hs-boot-subtree!
   (fn