WIP: pre-existing changes from WASM browser work + test infrastructure

Accumulated changes from WASM browser development sessions:
- sx_runtime.ml: signal subscription + notify, env unwrap tolerance
- sx_browser.bc.js: rebuilt js_of_ocaml browser kernel
- sx_browser.bc.wasm.js + assets: WASM browser kernel build
- sx-platform.js browser tests (test_js, test_platform, test_wasm)
- Playwright sx-inspect.js: interactive page inspector tool
- harness-web.sx: DOM assertion updates
- deploy.sh, Dockerfile, dune-project: build config updates
- test-stepper.sx: stepper unit tests
- reader-macro-demo plan update

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

hosts/ocaml/browser/test_js.js

@@ -0,0 +1,62 @@
+#!/usr/bin/env node
+// Test js_of_ocaml build of SX kernel
+const path = require('path');
+require(path.join(__dirname, '../_build/default/browser/sx_browser.bc.js'));
+
+const sx = globalThis.SxKernel;
+console.log('Engine:', sx.engine());
+
+const tests = [
+  ['(+ 1 2)', 3],
+  ['(- 10 3)', 7],
+  ['(* 6 7)', 42],
+  ['(/ 10 2)', 5],
+  ['(= 5 5)', true],
+  ['(< 3 5)', true],
+  ['(> 5 3)', true],
+  ['(not false)', true],
+  ['(inc 5)', 6],
+  ['(dec 5)', 4],
+  ['(len (list 1 2 3))', 3],
+  ['(len "hello")', 5],
+  ['(first (list 10 20))', 10],
+  ['(nth "hello" 0)', 'h'],
+  ['(nth "hello" 4)', 'o'],
+  ['(str "a" "b")', 'ab'],
+  ['(join ", " (list "a" "b" "c"))', 'a, b, c'],
+  ['(let ((x 10) (y 20)) (+ x y))', 30],
+  ['(if true "yes" "no")', 'yes'],
+  ['(cond (= 1 2) "one" :else "other")', 'other'],
+  ['(case 2 1 "one" 2 "two" :else "other")', 'two'],
+  ['(render-to-html (list (quote div) "hello"))', '<div>hello</div>'],
+  ['(render-to-html (list (quote span) (list (quote b) "bold")))', '<span><b>bold</b></span>'],
+  // Lambda + closure
+  ['(let ((add (fn (a b) (+ a b)))) (add 3 4))', 7],
+  ['(let ((x 10)) (let ((f (fn () x))) (f)))', 10],
+  // Higher-order
+  ['(len (filter (fn (x) (> x 2)) (list 1 2 3 4 5)))', 3],
+  // Recursion
+  ['(let ((fact (fn (n) (if (<= n 1) 1 (* n (fact (- n 1))))))) (fact 5))', 120],
+];
+
+let passed = 0, failed = 0;
+for (const [expr, expected] of tests) {
+  try {
+    const result = sx.eval(expr);
+    const ok = typeof expected === 'object'
+      ? result && result._type === expected._type
+      : result === expected;
+    if (ok) {
+      passed++;
+    } else {
+      console.log(`  FAIL: ${expr} = ${JSON.stringify(result)} (expected ${JSON.stringify(expected)})`);
+      failed++;
+    }
+  } catch (e) {
+    console.log(`  ERROR: ${expr}: ${e.message || e}`);
+    failed++;
+  }
+}
+
+console.log(`${passed} passed, ${failed} failed`);
+process.exit(failed > 0 ? 1 : 0);