Atomic island hydration: replaceChildren instead of clear+append

The hydrate-island function was doing:
  (dom-set-text-content el "")  ;; clears SSR content — visible flash
  (dom-append el body-dom)       ;; adds reactive DOM

Now uses:
  (host-call el "replaceChildren" body-dom)  ;; atomic swap, no empty state

Per DOM spec, replaceChildren is a single synchronous operation — the
browser never renders the intermediate empty state. The MutationObserver
test now checks for content going to zero (visible gap), not mutation
count (mutations are expected during any swap).

Test: "No clobber: clean" — island never goes empty during hydration.
All 8 home features pass: no-flash, no-clobber, boot, islands, stepper,
smoke, no-errors.

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

web/boot.sx

@@ -331,8 +331,7 @@
                 (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)))))
-                (dom-set-text-content el "")
-                (dom-append el body-dom)
+                (host-call el "replaceChildren" body-dom)
                 (dom-set-data el "sx-disposers" disposers)
                 (set-timeout (fn () (process-elements el)) 0)
                 (log-info