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>

hosts/ocaml/browser/bundle.sh

@@ -27,6 +27,7 @@ cp "$BUILD/sx_browser.bc.js" "$DIST/"
 cp sx-platform.js "$DIST/"
 
 # 3. Spec modules
+cp "$ROOT/spec/signals.sx"     "$DIST/sx/core-signals.sx"
 cp "$ROOT/spec/render.sx"      "$DIST/sx/"
 cp "$ROOT/web/signals.sx"      "$DIST/sx/"
 cp "$ROOT/web/deps.sx"         "$DIST/sx/"