Remove reactive class/style (CSSX covers it), add error boundaries + resource

Reactive class/style (:class-map, :style-map) removed — CSSX components
already handle dynamic class/style via defcomp with full SX logic.

Added:
- error-boundary render-dom form: try/catch around children, renders
  fallback fn with (err retry) on failure, disposes partial effects
- resource async signal: wraps promise into signal with loading/data/error
  states, transitions automatically on resolve/reject
- try-catch, error-message, promise-then platform functions
- Updated Phase 2 status tables and demo page numbering

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

shared/sx/ref/signals.sx

@@ -408,3 +408,33 @@
                   (reset! target-signal new-val))))))
         ;; Return cleanup — removes listener on dispose/re-run
         remove)))))
+
+
+;; ==========================================================================
+;; 15. Resource — async signal with loading/resolved/error states
+;; ==========================================================================
+;;
+;; A resource wraps an async operation (fetch, computation) and exposes
+;; its state as a signal. The signal transitions through:
+;;   {:loading true  :data nil    :error nil}  — initial/loading
+;;   {:loading false :data result :error nil}  — success
+;;   {:loading false :data nil    :error err}  — failure
+;;
+;; Usage:
+;;   (let ((user (resource (fn () (fetch-json "/api/user")))))
+;;     (cond
+;;       (get (deref user) "loading")  (div "Loading...")
+;;       (get (deref user) "error")    (div "Error: " (get (deref user) "error"))
+;;       :else                         (div (get (deref user) "data"))))
+;;
+;; Platform interface required:
+;;   (promise-then promise on-resolve on-reject) → void
+
+(define resource
+  (fn (fetch-fn)
+    (let ((state (signal (dict "loading" true "data" nil "error" nil))))
+      ;; Kick off the async operation
+      (promise-then (invoke fetch-fn)
+        (fn (data) (reset! state (dict "loading" false "data" data "error" nil)))
+        (fn (err) (reset! state (dict "loading" false "data" nil "error" err))))
+      state)))