Fix env-shadowing: rebind host extension points after .sx file load

evaluator.sx defines *custom-special-forms* and register-special-form!
which shadow the host's native bindings when loaded at runtime. The
native bindings route to Sx_ref.custom_special_forms (the dict the CEK
evaluator checks), but the SX-level defines create a separate dict.

Fix: rebind_host_extensions runs after every load command, re-asserting
the native register-special-form! and *custom-special-forms* bindings.

Add regression test: custom form registered before evaluator.sx load
survives and remains callable via CEK dispatch afterward.

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

run-tests.sh

@@ -101,6 +101,16 @@ check('defhandler via eval', '(has-key? (defhandler test-h (&key x) x) \"__type\
 check('definition-form-extensions populated', '(> (len *definition-form-extensions*) 0)', 'true')
 check('RENDER_HTML_FORMS has defstyle', '(contains? RENDER_HTML_FORMS \"defstyle\")', 'true')
 
+# Env-shadowing regression: custom forms survive evaluator.sx load
+bridge2 = OcamlSync()
+bridge2.eval('(register-special-form! \"shadow-test\" (fn (args env) 42))')
+bridge2.load('spec/evaluator.sx')
+check('custom form survives evaluator.sx load',
+      bridge2.eval('(has-key? *custom-special-forms* \"shadow-test\")'), 'true')
+bridge2.eval('(register-special-form! \"post-load\" (fn (args env) 99))')
+check('custom form callable after evaluator.sx load',
+      bridge2.eval('(post-load 1)'), '99')
+
 print(f'\\nResults: {ok} passed, {fail} failed')
 import sys; sys.exit(1 if fail > 0 else 0)
 "