Add suspense, resource, and transitions — Phase 2 complete

- suspense render-dom form: shows fallback while resource loads, swaps
  to body content when resource signal resolves
- resource async signal: wraps promise into signal with loading/data/error
  dict, auto-transitions on resolve/reject via promise-then
- transition: defers signal writes to requestIdleCallback, sets pending
  signal for UI feedback during expensive operations
- Added schedule-idle, promise-then platform functions
- All Phase 2 features now marked Done in status tables

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/adapter-dom.sx

@@ -309,7 +309,8 @@
 (define RENDER_DOM_FORMS
   (list "if" "when" "cond" "case" "let" "let*" "begin" "do"
         "define" "defcomp" "defisland" "defmacro" "defstyle" "defhandler"
-        "map" "map-indexed" "filter" "for-each" "portal" "error-boundary"))
+        "map" "map-indexed" "filter" "for-each" "portal"
+        "error-boundary" "suspense"))
 
 (define render-dom-form?
   (fn (name)
@@ -434,6 +435,10 @@
       (= name "error-boundary")
         (render-dom-error-boundary args env ns)
 
+      ;; suspense — show fallback while resource is loading
+      (= name "suspense")
+        (render-dom-suspense args env ns)
+
       ;; for-each (render variant)
       (= name "for-each")
         (let ((f (trampoline (eval-expr (nth expr 1) env)))
@@ -823,6 +828,66 @@
         container))))
 
 
+;; --------------------------------------------------------------------------
+;; render-dom-suspense — show fallback while resource is loading
+;; --------------------------------------------------------------------------
+;;
+;; (suspense fallback-expr body...)
+;;
+;; Renders fallback-expr initially. When used with a resource signal,
+;; an effect watches the resource state and swaps in the body content
+;; once loading is complete. If the resource errors, renders the error.
+;;
+;; The simplest pattern: wrap a resource deref in suspense.
+;;
+;; (suspense
+;;   (div "Loading...")
+;;   (let ((data (get (deref user-resource) "data")))
+;;     (div (get data "name"))))
+
+(define render-dom-suspense
+  (fn (args env ns)
+    (let ((fallback-expr (first args))
+          (body-exprs (rest args))
+          (container (dom-create-element "div" nil)))
+      (dom-set-attr container "data-sx-suspense" "true")
+      ;; Render fallback immediately
+      (dom-append container (render-to-dom fallback-expr env ns))
+      ;; Try to render body — if it works, replace fallback
+      ;; The body typically derefs a resource signal, which triggers
+      ;; an effect that re-renders when the resource resolves
+      (let ((body-disposers (list)))
+        (effect (fn ()
+          ;; Dispose previous body renders
+          (for-each (fn (d) (d)) body-disposers)
+          (set! body-disposers (list))
+          ;; Try rendering the body
+          (try-catch
+            (fn ()
+              (let ((frag (create-fragment)))
+                (with-island-scope
+                  (fn (disposable)
+                    (append! body-disposers disposable)
+                    (register-in-scope disposable))
+                  (fn ()
+                    (for-each
+                      (fn (child)
+                        (dom-append frag (render-to-dom child env ns)))
+                      body-exprs)))
+                ;; Success — replace container content with body
+                (dom-set-prop container "innerHTML" "")
+                (dom-append container frag)))
+            (fn (err)
+              ;; Body threw — keep showing fallback (or show error)
+              nil))))
+        ;; Register cleanup
+        (register-in-scope
+          (fn ()
+            (for-each (fn (d) (d)) body-disposers)
+            (set! body-disposers (list)))))
+      container)))
+
+
 ;; --------------------------------------------------------------------------
 ;; Platform interface — DOM adapter
 ;; --------------------------------------------------------------------------