Fix thunk handling for island SSR + effect no-op on server

- trampoline resolves Thunk values (sf-letrec returns them for TCO)
- render-to-html handles "thunk" type by unwrapping expr+env
- effect overridden to no-op after loading signals.sx (prevents
  reactive loops during SSR — effects are DOM side-effects)
- Added thunk?/thunk-expr/thunk-env primitives
- Added DOM API stubs for SSR (dom-query, schedule-idle, etc.)

Header island renders fully with styling. Stepper island still
fails SSR (letrec + complex body hits "Undefined symbol: div"
in eval path — render mode not active during CEK letrec eval).

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

shared/sx/ocaml_bridge.py

@@ -439,6 +439,15 @@ class OcamlBridge:
                         skipped += 1
                         _logger.warning("OCaml load skipped %s: %s",
                                         filepath, e)
+                # SSR overrides: effect is a no-op on the server (prevents
+                # reactive loops during island SSR — effects are DOM side-effects)
+                try:
+                    noop_dispose = '(fn () nil)'
+                    await self._send(f'(load-source "(define effect (fn (f) {noop_dispose}))")')
+                    await self._read_until_ok(ctx=None)
+                except OcamlBridgeError:
+                    pass
+
                 # Register JIT hook — lambdas compile on first call
                 try:
                     await self._send('(vm-compile-adapter)')