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_wasm.js

@@ -0,0 +1,73 @@
+#!/usr/bin/env node
+// Test WASM build of SX kernel
+const path = require('path');
+const build_dir = path.join(__dirname, '../_build/default/browser');
+
+async function main() {
+  // Load WASM module — require.main.filename must point to build dir
+  // so the WASM loader finds .wasm assets via path.dirname(require.main.filename)
+  require.main.filename = path.join(build_dir, 'test_wasm.js');
+  require(path.join(build_dir, 'sx_browser.bc.wasm.js'));
+
+  // Wait for WASM init
+  await new Promise(r => setTimeout(r, 2000));
+
+  const sx = globalThis.SxKernel;
+  if (!sx) {
+    console.error('FAIL: SxKernel not available');
+    process.exit(1);
+  }
+
+  console.log('Engine:', sx.engine());
+
+  // Basic tests
+  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>'],
+    ['(let ((add (fn (a b) (+ a b)))) (add 3 4))', 7],
+    ['(let ((x 10)) (let ((f (fn () x))) (f)))', 10],
+    ['(len (filter (fn (x) (> x 2)) (list 1 2 3 4 5)))', 3],
+    ['(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) {
+    const result = sx.eval(expr);
+    const ok = typeof expected === 'object'
+      ? result && result._type === expected._type
+      : result === expected;
+    if (ok) {
+      console.log(`  PASS: ${expr} = ${JSON.stringify(result)}`);
+      passed++;
+    } else {
+      console.log(`  FAIL: ${expr} = ${JSON.stringify(result)} (expected ${JSON.stringify(expected)})`);
+      failed++;
+    }
+  }
+
+  console.log(`\n${passed} passed, ${failed} failed`);
+  process.exit(failed > 0 ? 1 : 0);
+}
+
+main().catch(e => { console.error(e); process.exit(1); });