Split signals: core spec (spec/signals.sx) + web extensions (web/signals.sx)

Core reactive primitives (signal, deref, reset!, swap!, computed, effect,
batch, notify-subscribers, dispose-computed, with-island-scope,
register-in-scope) moved to spec/signals.sx — pure SX, zero platform
dependencies. Usable by any host: web, CLI, embedded, server, harness.

Web extensions (marsh scopes, stores, event bridge, resource) remain in
web/signals.sx, which now depends on spec/signals.sx.

Updated all load paths:
- hosts/ocaml/bin/sx_server.ml — loads spec/signals.sx before web/signals.sx
- hosts/ocaml/bin/run_tests.ml — loads both in order
- hosts/ocaml/browser/bundle.sh + sx-platform.js — loads core-signals.sx first
- shared/sx/ocaml_bridge.py — loads spec/signals.sx before web extensions

1116/1116 OCaml tests pass. Browser reactive island preview works.

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

shared/sx/ocaml_bridge.py

@@ -384,7 +384,14 @@ class OcamlBridge:
             # All directories loaded into the Python env
             all_dirs = list(set(_watched_dirs) | _dirs_from_cache)
 
-            # Isomorphic libraries: signals, rendering, web forms
+            # Core spec: signals (must load before web/signals.sx extensions)
+            spec_dir = os.path.join(os.path.dirname(__file__), "../../spec")
+            if os.path.isdir(spec_dir):
+                for spec_file in ["signals.sx"]:
+                    path = os.path.normpath(os.path.join(spec_dir, spec_file))
+                    if os.path.isfile(path):
+                        all_files.append(path)
+            # Isomorphic libraries: signals extensions, rendering, web forms
             web_dir = os.path.join(os.path.dirname(__file__), "../../web")
             if os.path.isdir(web_dir):
                 for web_file in ["signals.sx", "adapter-html.sx", "adapter-sx.sx",