WASM browser build: interned env keys, async kernel boot, bundled .sx platform

- Symbol interning in sx_types.ml: env lookups use int keys (intern/unintern)
  to avoid repeated string hashing in scope chain walks
- sx-platform.js: poll for SxKernel availability (WASM init is async)
- shell.sx: load sx_browser.bc.wasm.js when SX_USE_WASM=1
- bundle.sh: fix .sx file paths (web-signals.sx rename)
- browser/dune: target byte+js+wasm modes
- Bundle 23 .sx platform files for browser (dom, signals, router, boot, etc.)

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

shared/static/wasm/sx/signals.sx

@@ -0,0 +1,97 @@
+;; ==========================================================================
+;; web/signals.sx — Web platform signal extensions
+;;
+;; Extends the core reactive signal spec (spec/signals.sx) with web-specific
+;; features: marsh scopes (DOM lifecycle), named stores (page-level state),
+;; event bridge (lake→island communication), and async resources.
+;;
+;; These depend on platform primitives:
+;;   dom-set-data, dom-get-data, dom-listen, dom-dispatch, event-detail,
+;;   promise-then
+;; ==========================================================================
+
+
+;; --------------------------------------------------------------------------
+;; Marsh scopes — child scopes within islands
+;; --------------------------------------------------------------------------
+
+(define with-marsh-scope :effects [mutation io]
+  (fn (marsh-el (body-fn :as lambda))
+    (let ((disposers (list)))
+      (with-island-scope
+        (fn (d) (append! disposers d))
+        body-fn)
+      (dom-set-data marsh-el "sx-marsh-disposers" disposers))))
+
+(define dispose-marsh-scope :effects [mutation io]
+  (fn (marsh-el)
+    (let ((disposers (dom-get-data marsh-el "sx-marsh-disposers")))
+      (when disposers
+        (for-each (fn ((d :as lambda)) (cek-call d nil)) disposers)
+        (dom-set-data marsh-el "sx-marsh-disposers" nil)))))
+
+
+;; --------------------------------------------------------------------------
+;; Named stores — page-level signal containers
+;; --------------------------------------------------------------------------
+
+(define *store-registry* (dict))
+
+(define def-store :effects [mutation]
+  (fn ((name :as string) (init-fn :as lambda))
+    (let ((registry *store-registry*))
+      (when (not (has-key? registry name))
+        (set! *store-registry* (assoc registry name (cek-call init-fn nil))))
+      (get *store-registry* name))))
+
+(define use-store :effects []
+  (fn ((name :as string))
+    (if (has-key? *store-registry* name)
+      (get *store-registry* name)
+      (error (str "Store not found: " name
+                  ". Call (def-store ...) before (use-store ...).")))))
+
+(define clear-stores :effects [mutation]
+  (fn ()
+    (set! *store-registry* (dict))))
+
+
+;; --------------------------------------------------------------------------
+;; Event bridge — DOM event communication for lake→island
+;; --------------------------------------------------------------------------
+
+(define emit-event :effects [io]
+  (fn (el (event-name :as string) detail)
+    (dom-dispatch el event-name detail)))
+
+(define on-event :effects [io]
+  (fn (el (event-name :as string) (handler :as lambda))
+    (dom-on el event-name handler)))
+
+(define bridge-event :effects [mutation io]
+  (fn (el (event-name :as string) (target-signal :as signal) transform-fn)
+    (effect (fn ()
+      (let ((remove (dom-on el event-name
+              (fn (e)
+                (let ((detail (event-detail e))
+                      (new-val (if transform-fn
+                                 (cek-call transform-fn (list detail))
+                                 detail)))
+                  (reset! target-signal new-val))))))
+        remove)))))
+
+
+;; --------------------------------------------------------------------------
+;; Resource — async signal with loading/resolved/error states
+;; --------------------------------------------------------------------------
+
+(define resource :effects [mutation io]
+  (fn ((fetch-fn :as lambda))
+    (let ((state (signal (dict "loading" true "data" nil "error" nil))))
+      (promise-then
+        (cek-call fetch-fn nil)
+        (fn (data)
+          (reset! state (dict "loading" false "data" data "error" nil)))
+        (fn (err)
+          (reset! state (dict "loading" false "data" nil "error" err))))
+      state)))