Add failing tests: set! in named let loop (WASM + native OCaml)

Named let creates a loop continuation but set! inside the loop body
does not mutate bindings in the enclosing let scope. Affects both
the WASM kernel and native OCaml CEK evaluator.

6 failing Node tests cover:
- set! counter (simplest case)
- set! counter with named let params
- set! list accumulator via append
- append! + set! counter combo
- set! string concatenation
- nested named let set!

3 baselines pass: plain let set!, functional named let, plain append!

Also adds spec/tests/test-named-let-set.sx (7 assertions, first
fails and aborts — confirms bug exists in spec test suite too).

This is the root cause of empty source code blocks on all example
pages: tokenize-sx uses set! in named let → empty tokens →
highlight returns "(<> )" → empty <code> blocks.

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

spec/tests/test-named-let-set.sx

@@ -0,0 +1,52 @@
+(assert=
+  (let ((x 0)) (let loop () (when (< x 5) (set! x (+ x 1)) (loop))) x)
+  5
+  "set! counter in named let loop")
+
+(assert=
+  (let
+    ((acc (list)))
+    (let
+      loop
+      ((i 0))
+      (when (< i 3) (set! acc (append acc (list i))) (loop (+ i 1))))
+    acc)
+  (list 0 1 2)
+  "set! list accumulator in named let with param")
+
+(assert=
+  (let
+    ((acc (list)) (i 0))
+    (let loop () (when (< i 3) (append! acc i) (set! i (+ i 1)) (loop)))
+    acc)
+  (list 0 1 2)
+  "append! + set! counter in named let")
+
+(assert=
+  (let
+    ((sum 0))
+    (let
+      loop
+      ((i 1))
+      (when (<= i 10) (set! sum (+ sum i)) (loop (+ i 1))))
+    sum)
+  55
+  "set! sum in named let loop (1..10)")
+
+(assert=
+  (let
+    ((result ""))
+    (let
+      loop
+      ((i 0))
+      (when (< i 3) (set! result (str result (str i))) (loop (+ i 1))))
+    result)
+  "012"
+  "set! string accumulator in named let")
+
+(assert= (let ((x 0)) (set! x 42) x) 42 "set! in plain let (baseline)")
+
+(assert=
+  (let loop ((i 0) (acc 0)) (if (< i 5) (loop (+ i 1) (+ acc i)) acc))
+  10
+  "named let without set! (baseline)")

tests/node/test-named-let-set.js

@@ -0,0 +1,82 @@
+#!/usr/bin/env node
+/**
+ * test-named-let-set.js — WASM kernel tests for set! inside named let loops.
+ *
+ * FAILING: set! inside named let body does not persist mutations to
+ * the enclosing scope. This breaks tokenize-sx → highlight → source display.
+ */
+const { createSxEnv } = require('./sx-harness');
+
+let passed = 0, failed = 0;
+const origLog = console.log;
+const origErr = console.error;
+
+function test(name, expr, expected) {
+  let env;
+  try {
+    // Fresh env per test to avoid cross-contamination
+    // But creating env is slow, so we'll reuse one below
+  } catch(e) {}
+}
+
+async function main() {
+  origLog('=== Named Let + set! Tests (WASM kernel) ===\n');
+  const t0 = Date.now();
+
+  const env = await createSxEnv({});
+
+  function check(name, expr, expected) {
+    const result = env.eval(expr);
+    const expectedStr = JSON.stringify(expected);
+    const resultVal = result?.items ? result.items : result;
+    const resultStr = JSON.stringify(resultVal);
+    if (resultStr === expectedStr) {
+      passed++;
+      origLog(`  \u2713 ${name}`);
+    } else {
+      failed++;
+      origErr(`  \u2717 ${name}: expected ${expectedStr}, got ${resultStr}`);
+    }
+  }
+
+  // ---- Baselines (should pass) ----
+  origLog('Baselines:');
+  check('set! in plain let',
+    '(let ((x 0)) (set! x 42) x)', 42);
+
+  check('named let without set! (functional accumulator)',
+    '(trampoline (let loop ((i 0) (acc 0)) (if (< i 5) (loop (+ i 1) (+ acc i)) acc)))', 10);
+
+  check('append! on outer binding (no named let)',
+    '(let ((xs (list))) (append! xs 1) (append! xs 2) xs)', [1, 2]);
+
+  // ---- Named let + set! (the broken cases) ----
+  origLog('\nNamed let + set!:');
+  check('set! counter in named let loop',
+    '(let ((x 0)) (let loop () (when (< x 5) (set! x (+ x 1)) (loop))) x)', 5);
+
+  check('set! counter in named let loop (with param)',
+    '(let ((total 0)) (let loop ((i 1)) (when (<= i 10) (set! total (+ total i)) (loop (+ i 1)))) total)', 55);
+
+  check('set! list via append in named let',
+    '(let ((acc (list))) (let loop ((i 0)) (when (< i 3) (set! acc (append acc (list i))) (loop (+ i 1)))) acc)', [0, 1, 2]);
+
+  check('append! in named let',
+    '(let ((acc (list)) (i 0)) (let loop () (when (< i 3) (append! acc i) (set! i (+ i 1)) (loop))) acc)', [0, 1, 2]);
+
+  check('set! string concat in named let',
+    '(let ((result "")) (let loop ((i 0)) (when (< i 3) (set! result (str result (str i))) (loop (+ i 1)))) result)', "012");
+
+  // ---- Nested named let ----
+  origLog('\nNested:');
+  check('set! in nested named let',
+    '(let ((count 0)) (let outer ((i 0)) (when (< i 3) (let inner ((j 0)) (when (< j 2) (set! count (+ count 1)) (inner (+ j 1)))) (outer (+ i 1)))) count)', 6);
+
+  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); });