WIP: DOM-preserving hydration — SSR DOM stays, no visual flash

Adds hydration cursor to render pipeline:
- boot.sx: *hydrating* flag, hydrate-start!/stop!, cursor stack helpers
- adapter-dom.sx: render-dom-element uses existing SSR elements when
  *hydrating* is true. Text nodes reused. dom-append skipped.
- hydrate-island: calls hydrate-start! before render-to-dom, no
  replaceChildren. SSR DOM stays in place.

Status: screenshots identical (no visual flash), but event listeners
not attaching — the cursor/set! interaction between CEK and VM needs
debugging. The hydrate-start! set! on *hydrating* may not propagate
to the bytecoded adapter-dom render path.

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

web/boot.sx

@@ -55,6 +55,8 @@
                 (dom-append-to-head el)))))
         els))))
 
+(define *hydrating* false)
+
 (define
   sx-mount
   :effects (mutation io)
@@ -329,9 +331,14 @@
                     p
                     (if (dict-has? kwargs p) (dict-get kwargs p) nil)))
                 (component-params comp))
-              (let
-                ((body-dom (cek-try (fn () (with-island-scope (fn (disposable) (append! disposers disposable)) (fn () (render-to-dom (component-body comp) local nil)))) (fn (err) (log-warn (str "hydrate-island FAILED: " comp-name " — " err)) (let ((error-el (dom-create-element "div" nil))) (dom-set-attr error-el "class" "sx-island-error") (dom-set-attr error-el "style" "padding:8px;margin:4px 0;border:1px solid #ef4444;border-radius:4px;background:#fef2f2;color:#b91c1c;font-family:monospace;font-size:12px;white-space:pre-wrap") (dom-set-text-content error-el (str "Island error: " comp-name "\n" err)) error-el)))))
-                (host-call el "replaceChildren" body-dom)
+              (do
+                (hydrate-start! el)
+                (log-info "  hydrate-start done")
+                (with-island-scope
+                  (fn (disposable) (append! disposers disposable))
+                  (fn () (render-to-dom (component-body comp) local nil)))
+                (log-info "  render-to-dom done")
+                (hydrate-stop!)
                 (dom-set-data el "sx-disposers" disposers)
                 (set-timeout (fn () (process-elements el)) 0)
                 (log-info