Fix signal-add-sub! losing subscribers after remove, fix build pipeline

signal-add-sub! used (append! subscribers f) which returns a new list
for immutable List but discards the result — after signal-remove-sub!
replaces the subscribers list via dict-set!, re-adding subscribers
silently fails. Counter island only worked once (0→1 then stuck).

Fix: use (dict-set! s "subscribers" (append ...)) to explicitly update
the dict field, matching signal-remove-sub!'s pattern.

Build pipeline fixes:
- sx-build-all.sh now bundles spec→dist and recompiles .sxbc bytecode
- compile-modules.js syncs .sx source files alongside .sxbc to wasm/sx/
- Per-file cache busting: wasm, platform JS, and sxbc each get own hash
- bundle.sh adds cssx.sx to dist

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

shared/sx/templates/shell.sx

@@ -13,6 +13,8 @@
     (body-html :as string?)
     (asset-url :as string)
     (wasm-hash :as string?)
+    (platform-hash :as string?)
+    (sxbc-hash :as string?)
     (inline-css :as string?)
     (inline-head-js :as string?)
     (init-sx :as string?))
@@ -74,8 +76,12 @@
           :type "text/sx"
           :data-mount "#sx-root"
           (raw! (or page-sx "")))
-        (let
-          ((wv (or wasm-hash "0")))
-          (<>
-            (script :src (str asset-url "/wasm/sx_browser.bc.wasm.js?v=" wv))
-            (script :src (str asset-url "/wasm/sx-platform-2.js?v=" wv))))))))
+        (<>
+          (script
+            :src (str
+              asset-url
+              "/wasm/sx_browser.bc.wasm.js?v="
+              (or wasm-hash "0")))
+          (script
+            :src (str asset-url "/wasm/sx-platform-2.js?v=" (or platform-hash "0"))
+            :data-sxbc-hash (or sxbc-hash "0")))))))