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/bundle.sh

@@ -65,6 +65,9 @@ cp "$ROOT/web/engine.sx"        "$DIST/sx/"
 cp "$ROOT/web/orchestration.sx" "$DIST/sx/"
 cp "$ROOT/web/boot.sx"          "$DIST/sx/"
 
+# 9. CSSX (stylesheet language)
+cp "$ROOT/sx/sx/cssx.sx"        "$DIST/sx/"
+
 # Summary
 WASM_SIZE=$(du -sh "$DIST/sx_browser.bc.wasm.assets" | cut -f1)
 JS_SIZE=$(du -sh "$DIST/sx_browser.bc.js" | cut -f1)