DOM-preserving hydration — SSR DOM stays, event listeners attach in place

Scope-based cursor walks the existing SSR DOM during island hydration
instead of creating new elements and calling replaceChildren. The
hydration scope (sx-hydrating) propagates through define-library via
scope-push!/peek/pop!, solving the env isolation that broke the
previous set!-based approach.

Changes:
- adapter-dom.sx: hydrating?, hydrate-next-node, hydrate-enter/exit-element
  helpers. render-to-dom reuses text nodes. render-dom-element reuses
  elements by tag match, skips dom-append. reactive-text/cek-reactive-text
  reuse existing text nodes. render-dom-fragment/lake/marsh skip append.
  dispatch-render-form (if/when/cond) injects markers into existing DOM.
- boot.sx: hydrate-island pushes cursor scope, skips replaceChildren.
  On mismatch error, falls back to full re-render.

Result: zero DOM destruction, zero visual flash, event listeners
attached to original SSR elements. Stepper clicks verified working.

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

shared/static/wasm/sx/boot.sx

@@ -330,13 +330,29 @@
                     (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)
+                ((cursor (dict "parent" el "index" 0)))
+                (scope-push! "sx-hydrating" cursor)
+                (cek-try
+                  (fn
+                    ()
+                    (with-island-scope
+                      (fn (disposable) (append! disposers disposable))
+                      (fn () (render-to-dom (component-body comp) local nil))))
+                  (fn
+                    (err)
+                    (scope-pop! "sx-hydrating")
+                    (log-warn
+                      (str "hydrate fallback: " comp-name " — " err))
+                    (let
+                      ((fallback (cek-try (fn () (with-island-scope (fn (d) (append! disposers d)) (fn () (render-to-dom (component-body comp) local nil)))) (fn (err2) (let ((e (dom-create-element "div" nil))) (dom-set-text-content e (str "Island error: " comp-name "\n" err2)) e)))))
+                      (host-call el "replaceChildren" fallback)
+                      nil)))
+                (scope-pop! "sx-hydrating")
                 (dom-set-data el "sx-disposers" disposers)
                 (set-timeout (fn () (process-elements el)) 0)
                 (log-info
                   (str
-                    "hydrated island: "
+                    "hydrated island: ~"
                     comp-name
                     " ("
                     (len disposers)

shared/static/wasm/sx/module-manifest.json

@@ -536,6 +536,7 @@
       "SVG_NS",
       "MATH_NS",
       "island-scope?",
+      "hydrating?",
       "contains-deref?",
       "dom-on",
       "render-to-dom",

web/boot.sx

@@ -330,13 +330,29 @@
                     (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)
+                ((cursor (dict "parent" el "index" 0)))
+                (scope-push! "sx-hydrating" cursor)
+                (cek-try
+                  (fn
+                    ()
+                    (with-island-scope
+                      (fn (disposable) (append! disposers disposable))
+                      (fn () (render-to-dom (component-body comp) local nil))))
+                  (fn
+                    (err)
+                    (scope-pop! "sx-hydrating")
+                    (log-warn
+                      (str "hydrate fallback: " comp-name " — " err))
+                    (let
+                      ((fallback (cek-try (fn () (with-island-scope (fn (d) (append! disposers d)) (fn () (render-to-dom (component-body comp) local nil)))) (fn (err2) (let ((e (dom-create-element "div" nil))) (dom-set-text-content e (str "Island error: " comp-name "\n" err2)) e)))))
+                      (host-call el "replaceChildren" fallback)
+                      nil)))
+                (scope-pop! "sx-hydrating")
                 (dom-set-data el "sx-disposers" disposers)
                 (set-timeout (fn () (process-elements el)) 0)
                 (log-info
                   (str
-                    "hydrated island: "
+                    "hydrated island: ~"
                     comp-name
                     " ("
                     (len disposers)