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/static/wasm/sx-platform-2.js

@@ -180,8 +180,12 @@
 
   var _baseUrl = "";
 
-  // Detect base URL from current script
+  // Detect base URL and cache-bust params from current script tag.
+  // _cacheBust comes from the script's own ?v= query string (used for .sx source fallback).
+  // _sxbcCacheBust comes from data-sxbc-hash attribute — a separate content hash
+  // covering all .sxbc files so each file gets its own correct cache buster.
   var _cacheBust = "";
+  var _sxbcCacheBust = "";
   (function() {
     if (typeof document !== "undefined") {
       var scripts = document.getElementsByTagName("script");
@@ -191,6 +195,8 @@
           _baseUrl = src.substring(0, src.lastIndexOf("/") + 1);
           var qi = src.indexOf("?");
           if (qi !== -1) _cacheBust = src.substring(qi);
+          var sxbcHash = scripts[i].getAttribute("data-sxbc-hash");
+          if (sxbcHash) _sxbcCacheBust = "?v=" + sxbcHash;
           break;
         }
       }
@@ -239,7 +245,7 @@
 
     // Try .sxbc (SX s-expression format, loaded via load-sxbc primitive)
     var sxbcPath = path.replace(/\.sx$/, '.sxbc');
-    var sxbcUrl = _baseUrl + sxbcPath + _cacheBust;
+    var sxbcUrl = _baseUrl + sxbcPath + _sxbcCacheBust;
     try {
       var xhr2 = new XMLHttpRequest();
       xhr2.open("GET", sxbcUrl, false);