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/ocaml_bridge.py

@@ -190,7 +190,8 @@ class OcamlBridge:
         # Only inject small, safe values as kernel variables.
         # Large/complex blobs use placeholder tokens at render time.
         for key in ("component_hash", "sx_css_classes", "asset_url",
-                    "sx_js_hash", "body_js_hash", "wasm_hash"):
+                    "sx_js_hash", "body_js_hash", "wasm_hash",
+                    "platform_hash", "sxbc_hash"):
             val = static.get(key) or ""
             var = f"__shell-{key.replace('_', '-')}"
             defn = f'(define {var} "{_escape(str(val))}")'
@@ -269,6 +270,7 @@ class OcamlBridge:
             placeholders = {}
             static_keys = {"component_hash", "sx_css_classes", "asset_url",
                            "sx_js_hash", "body_js_hash", "wasm_hash",
+                           "platform_hash", "sxbc_hash",
                            "head_scripts", "body_scripts"}
             # page_source is SX wire format that may contain \" escapes.
             # Send via binary blob protocol to avoid double-escaping