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/signals.sx

@@ -438,3 +438,29 @@
         (fn (data) (reset! state (dict "loading" false "data" data "error" nil)))
         (fn (err) (reset! state (dict "loading" false "data" nil "error" err))))
       state)))
+
+
+;; ==========================================================================
+;; 16. Transitions — non-urgent signal updates
+;; ==========================================================================
+;;
+;; Transitions mark updates as non-urgent. The thunk's signal writes are
+;; deferred to an idle callback, keeping the UI responsive during expensive
+;; computations.
+;;
+;; (transition pending-signal thunk)
+;;
+;; Sets pending-signal to true, schedules thunk via requestIdleCallback
+;; (or setTimeout 0 as fallback), then sets pending-signal to false when
+;; the thunk completes. Signal writes inside the thunk are batched.
+;;
+;; Platform interface required:
+;;   (schedule-idle thunk) → void  — requestIdleCallback or setTimeout(fn, 0)
+
+(define transition
+  (fn (pending-sig thunk)
+    (reset! pending-sig true)
+    (schedule-idle
+      (fn ()
+        (batch thunk)
+        (reset! pending-sig false)))))