Register component-affinity as JS primitive for runtime SX access

Fixes 4 test-eval.sx failures (component affinity tests).
Remaining 24 failures are server-only features (defpage, stream-*)
that don't belong in the browser evaluator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/bootstrap_js.py

@@ -1170,6 +1170,9 @@ CONTINUATIONS_JS = '''
 
   PRIMITIVES["continuation?"] = function(x) { return x != null && x._continuation === true; };
 
+  // Platform accessors — exposed as primitives so user SX code can call them
+  PRIMITIVES["component-affinity"] = componentAffinity;
+
   var _resetResume = [];
 
   function sfReset(args, env) {