Spec event bridge and named stores, move plan to reactive islands section

- signals.sx: add def-store/use-store/clear-stores (L3 named stores)
  and emit-event/on-event/bridge-event (lake→island DOM events)
- reactive-islands.sx: add event bridge, named stores, and plan pages
- Remove ~plan-reactive-islands-content from plans.sx
- Update nav-data.sx and docs.sx routing accordingly

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

shared/sx/ref/signals.sx

@@ -288,3 +288,87 @@
   (fn (disposable)
     (when *island-scope*
       (*island-scope* disposable))))
+
+
+;; ==========================================================================
+;; 12. Named stores — page-level signal containers (L3)
+;; ==========================================================================
+;;
+;; Stores persist across island creation/destruction. They live at page
+;; scope, not island scope. When an island is swapped out and re-created,
+;; it reconnects to the same store instance.
+;;
+;; The store registry is global page-level state. It survives island
+;; disposal but is cleared on full page navigation.
+
+(define *store-registry* (dict))
+
+(define def-store
+  (fn (name init-fn)
+    (let ((registry *store-registry*))
+      ;; Only create the store once — subsequent calls return existing
+      (when (not (has? registry name))
+        (set! *store-registry* (assoc registry name (init-fn))))
+      (get *store-registry* name))))
+
+(define use-store
+  (fn (name)
+    (if (has? *store-registry* name)
+      (get *store-registry* name)
+      (error (str "Store not found: " name
+                  ". Call (def-store ...) before (use-store ...).")))))
+
+(define clear-stores
+  (fn ()
+    (set! *store-registry* (dict))))
+
+
+;; ==========================================================================
+;; 13. Event bridge — DOM event communication for lake→island
+;; ==========================================================================
+;;
+;; Server-rendered content ("htmx lakes") inside reactive islands can
+;; communicate with island signals via DOM custom events. The bridge
+;; pattern:
+;;
+;;   1. Server renders a button/link with data-sx-emit="event-name"
+;;   2. When clicked, the client dispatches a CustomEvent on the element
+;;   3. The event bubbles up to the island container
+;;   4. An island effect listens for the event and updates signals
+;;
+;; This keeps server content pure HTML — no signal references needed.
+;; The island effect is the only reactive code.
+;;
+;; Platform interface required:
+;;   (dom-listen el event-name handler)   → remove-fn
+;;   (dom-dispatch el event-name detail)  → void
+;;   (event-detail e)                     → any
+;;
+;; These are platform primitives because they require browser DOM APIs.
+
+(define emit-event
+  (fn (el event-name detail)
+    (dom-dispatch el event-name detail)))
+
+(define on-event
+  (fn (el event-name handler)
+    (dom-listen el event-name handler)))
+
+;; Convenience: create an effect that listens for a DOM event on an
+;; element and writes the event detail (or a transformed value) into
+;; a target signal. Returns the effect's dispose function.
+;; When the effect is disposed (island teardown), the listener is
+;; removed automatically via the cleanup return.
+
+(define bridge-event
+  (fn (el event-name target-signal transform-fn)
+    (effect (fn ()
+      (let ((remove (dom-listen el event-name
+              (fn (e)
+                (let ((detail (event-detail e))
+                      (new-val (if transform-fn
+                                 (transform-fn detail)
+                                 detail)))
+                  (reset! target-signal new-val))))))
+        ;; Return cleanup — removes listener on dispose/re-run
+        remove)))))