hs: port loops/hs RPC test infrastructure to architecture's test runner

Two additions from loops/hs needed for the new WebSocket socket tests:
- unhandledRejection suppressor — synchronous test harness doesn't await RPC promises
- Fake setTimeout/clearTimeout + __hsFlushTimers — drain RPC timeout tests synchronously

Plan update: mark E36 WebSocket as DONE (previously "design-done, pending review").

Skipped: loops/hs's tests/playwright/generate-sx-tests.py — architecture's version
is 1468 lines vs loops/hs's 290; arch's is the further-evolved version.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/hs-run-filtered.js

@@ -14,6 +14,25 @@ const SX_DIR = path.join(WASM_DIR, 'sx');
 eval(fs.readFileSync(path.join(WASM_DIR, 'sx_browser.bc.js'), 'utf8'));
 const K = globalThis.SxKernel;
 
+process.on('unhandledRejection', () => {});
+
+let _fakeTimers = [];
+let _fakeTimerIdCtr = 0;
+const _realSetTimeout = globalThis.setTimeout;
+globalThis.setTimeout = function(cb, _delay) {
+  const id = ++_fakeTimerIdCtr;
+  _fakeTimers.push({ id, cb });
+  return id;
+};
+globalThis.clearTimeout = function(id) {
+  const idx = _fakeTimers.findIndex(t => t.id === id);
+  if (idx >= 0) _fakeTimers.splice(idx, 1);
+};
+globalThis.__hsFlushTimers = { call: function() {
+  const batch = _fakeTimers.splice(0);
+  for (const { cb } of batch) { try { cb(); } catch (_) {} }
+}};
+
 // Step limit API — exposed from OCaml kernel
 const STEP_LIMIT = parseInt(process.env.HS_STEP_LIMIT || '1000000');