js: fix lambda binding (index-of on lists), add vectors + R7RS platform stubs

- Fix PRIMITIVES["index-of"] for arrays: return NIL when not found (matching
  OCaml semantics) so bind-lambda-params correctly detects absent &rest params.
  Previously String(array).indexOf() returned -1, which passed number? check
  and mis-fired the &rest branch, leaving non-&rest params unbound.
- Declare var _lastErrorKont_ and var hostError in IIFE scope (strict mode fix)
- Add PRIMITIVES["host-error"], ["try-catch"], ["without-io-hook"]
- Add env["test-allowed?"] stub in run_tests.js
- Add spec/tests/test-vectors.sx: 42 tests for all vector primitives
- Rebuild sx-browser.js: 1847 standard / 2362 full tests pass (up from 5)

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

hosts/javascript/run_tests.js

@@ -293,6 +293,8 @@ env["pop-suite"] = function() {
   return null;
 };
 
+env["test-allowed?"] = function(name) { return true; };
+
 // Load test framework
 const projectDir = path.join(__dirname, "..", "..");
 const specTests = path.join(projectDir, "spec", "tests");