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>

hosts/ocaml/browser/compile-modules.js

@@ -185,6 +185,7 @@ const staticSxDir = path.resolve(__dirname, '..', '..', '..', 'shared', 'static'
 if (fs.existsSync(staticSxDir)) {
   let copied = 0;
   for (const file of FILES) {
+    // Copy bytecode
     for (const ext of ['.sxbc', '.sxbc.json']) {
       const src = path.join(sxDir, file.replace(/\.sx$/, ext));
       const dst = path.join(staticSxDir, file.replace(/\.sx$/, ext));
@@ -193,6 +194,13 @@ if (fs.existsSync(staticSxDir)) {
         copied++;
       }
     }
+    // Also sync .sx source files (fallback when .sxbc missing)
+    const sxSrc = path.join(sxDir, file);
+    const sxDst = path.join(staticSxDir, file);
+    if (fs.existsSync(sxSrc) && !fs.lstatSync(sxSrc).isSymbolicLink()) {
+      fs.copyFileSync(sxSrc, sxDst);
+      copied++;
+    }
   }
   console.log('Copied', copied, 'files to', staticSxDir);
 }