Store primitives + event-bridge + client? for isomorphic nav

- def-store/use-store/clear-stores: OCaml primitives with global
  mutable registry. Bypasses env scoping issues that prevented SX-level
  stores from persisting across bytecode module boundaries.

- client? primitive: _is_client ref (false on server, true in browser).
  Registered in primitives table for CALL_PRIM compatibility.

- Event-bridge island: rewritten to use document-level addEventListener
  via effect + host-callback, fixing container-ref timing issue.

- Header island: uses def-store for idx/shade signals when client? is
  true, plain signals when false (SSR compatibility).

- web-signals.sx: SX store definitions removed, OCaml primitives replace.

Isomorphic nav still fixme — client? works from K.eval but the JIT
"Not callable: nil" bug prevents proper primitive resolution during
render-to-dom hydration. Needs JIT investigation.

100 passed, 1 skipped, 0 failed.

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

tests/playwright/isomorphic.spec.js

@@ -177,7 +177,9 @@ test.describe('Isomorphic SSR', () => {
   });
 
   test.fixme('navigation preserves header island state', async ({ page }) => {
-    // BUG: def-store state not persisting — *store-registry* likely reset during SPA nav component reload
+    // BUG: client? primitive works from K.eval but not during render-to-dom hydration.
+    // Needs JIT/env investigation — the pre-existing JIT "Not callable: nil" bug
+    // prevents primitives registered after boot from being called during component eval.
     await page.goto(BASE_URL + '/sx/', { waitUntil: 'networkidle' });
 
     // Wait for header island to hydrate