hydrate-island: clear SSR children, render fresh DOM, append to island

Islands now: (1) clear SSR children via replaceChildren, (2) push nil
hydrating scope (disables hydration cursor walk that causes mismatch
errors), (3) render-to-dom creates fresh DOM with live event handlers,
(4) dom-append attaches the rendered DOM to the island element.

This fixes the hydrate-mismatch:div error caused by SSR/client attribute
differences (~tw generates different class strings server vs client).

NOTE: needs WASM rebuild (sx_build target=wasm) to compile boot.sxbc.
The .sx source is updated but the bytecoded module is stale.

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

shared/static/wasm/sx/boot.sx

@@ -332,13 +332,14 @@
                 (component-params comp))
               (let
                 ((cursor (dict "parent" el "index" 0)))
-                (scope-push! "sx-hydrating" cursor)
+                (host-call el "replaceChildren")
+                (scope-push! "sx-hydrating" nil)
                 (cek-try
                   (fn
                     ()
                     (with-island-scope
                       (fn (disposable) (append! disposers disposable))
-                      (fn () (render-to-dom (component-body comp) local nil))))
+                      (fn () (let ((body-dom (render-to-dom (component-body comp) local nil))) (when body-dom (dom-append el body-dom))))))
                   (fn
                     (err)
                     (scope-pop! "sx-hydrating")

web/boot.sx

@@ -332,13 +332,14 @@
                 (component-params comp))
               (let
                 ((cursor (dict "parent" el "index" 0)))
-                (scope-push! "sx-hydrating" cursor)
+                (host-call el "replaceChildren")
+                (scope-push! "sx-hydrating" nil)
                 (cek-try
                   (fn
                     ()
                     (with-island-scope
                       (fn (disposable) (append! disposers disposable))
-                      (fn () (render-to-dom (component-body comp) local nil))))
+                      (fn () (let ((body-dom (render-to-dom (component-body comp) local nil))) (when body-dom (dom-append el body-dom))))))
                   (fn
                     (err)
                     (scope-pop! "sx-hydrating")