Add failing tests: highlight returns empty due to set! in named let

Root cause: set! inside named let loop does not persist mutations
to the outer scope in the WASM kernel. tokenize-sx uses this pattern
(set! acc (append acc ...)) inside (let loop () ...) and gets empty
tokens. This makes highlight return "(<> )" for all inputs, causing
empty source code blocks on every reactive island example page.

6 failing tests document the bug at each level:
- set! in named let loop (root cause)
- tokenize-sx (empty tokens)
- highlight (empty output)
- component-source + highlight (end-to-end)

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

tests/node/test-highlight.js

@@ -0,0 +1,100 @@
+#!/usr/bin/env node
+/**
+ * test-highlight.js — Tests for the SX syntax highlighter.
+ *
+ * FAILING: highlight returns empty because set! inside named let loop
+ * does not persist mutations to the outer scope in the WASM kernel.
+ */
+const { createSxEnv } = require('./sx-harness');
+const fs = require('fs');
+const path = require('path');
+
+const HIGHLIGHT_SRC = path.resolve(__dirname, '../../lib/highlight.sx');
+
+let passed = 0, failed = 0;
+const origLog = console.log;
+const origErr = console.error;
+
+function assert(name, cond, detail) {
+  if (cond) { passed++; origLog(`  \u2713 ${name}`); }
+  else { failed++; origErr(`  \u2717 ${name}${detail ? ' — ' + detail : ''}`); }
+}
+
+async function main() {
+  origLog('=== Highlight Tests ===\n');
+  const t0 = Date.now();
+
+  // ---- Root cause: set! in named let loop ----
+  origLog('1. set! in named let loop');
+  {
+    const env = await createSxEnv({});
+    // This is the exact pattern tokenize-sx uses
+    const result = env.eval(
+      '(let ((acc (list)) (i 0)) (let loop () (when (< i 3) (set! acc (append acc (list i))) (set! i (+ i 1)) (loop))) acc)'
+    );
+    assert('named let set! persists', result?.items?.length === 3,
+      `expected 3 items, got ${result?.items?.length}`);
+    env.close();
+  }
+
+  // ---- Tokenizer ----
+  origLog('2. tokenize-sx');
+  {
+    const env = await createSxEnv({});
+    env.load(fs.readFileSync(HIGHLIGHT_SRC, 'utf8'));
+
+    const t1 = env.eval('(tokenize-sx "(+ 1 2)")');
+    assert('tokenize-sx non-empty', t1?.items?.length > 0,
+      `expected tokens, got ${t1?.items?.length}`);
+
+    const t2 = env.eval('(tokenize-sx "(define x 42)")');
+    assert('tokenize define', t2?.items?.length > 0,
+      `expected tokens, got ${t2?.items?.length}`);
+
+    env.close();
+  }
+
+  // ---- Highlight output ----
+  origLog('3. highlight');
+  {
+    const env = await createSxEnv({});
+    env.load(fs.readFileSync(HIGHLIGHT_SRC, 'utf8'));
+
+    const h1 = env.eval('(highlight "(+ 1 2)" "lisp")');
+    assert('highlight non-empty', h1 !== '(<> )' && h1?.length > 5,
+      `got "${h1}"`);
+
+    const h2 = env.eval('(highlight "(defisland ~counter () (div))" "lisp")');
+    assert('highlight defisland', h2?.includes?.('defisland'),
+      `got "${h2?.substring(0, 100)}"`);
+
+    env.close();
+  }
+
+  // ---- End-to-end: component-source + highlight ----
+  origLog('4. component-source + highlight');
+  {
+    const env = await createSxEnv({});
+    env.load(fs.readFileSync(HIGHLIGHT_SRC, 'utf8'));
+    env.load(fs.readFileSync(path.resolve(__dirname, '../../sx/sx/reactive-islands/index.sx'), 'utf8'));
+
+    // component-source needs pretty-print (server-only), test with manual source
+    const source = env.eval('(let ((c ~reactive-islands/index/demo-counter)) (str "(defisland ~" (component-name c) " " (component-params c) " " (component-body c) ")"))');
+    assert('component source string', typeof source === 'string' && source.length > 20,
+      `got ${typeof source}: "${source?.substring?.(0, 50)}"`);
+
+    if (typeof source === 'string' && source.length > 20) {
+      const highlighted = env.eval(`(highlight "${source.replace(/"/g, '\\"').replace(/\n/g, '\\n')}" "lisp")`);
+      assert('highlighted source non-empty', highlighted !== '(<> )' && highlighted?.length > 10,
+        `got "${highlighted?.substring?.(0, 100)}"`);
+    }
+
+    env.close();
+  }
+
+  const dt = Date.now() - t0;
+  origLog(`\n=== ${passed} passed, ${failed} failed (${dt}ms) ===`);
+  process.exit(failed > 0 ? 1 : 0);
+}
+
+main().catch(e => { origErr(e); process.exit(1); });