Add SX test harness: mock IO platform for testing components

spec/harness.sx — spec-level test harness with:
- Mock platform (30+ default IO mocks: fetch, query, DOM, storage, etc.)
- Session management (make-harness, harness-reset!, harness-set!/get)
- IO interception (make-interceptor, install-interceptors)
- IO log queries (io-calls, io-call-count, io-call-nth, io-call-args)
- IO assertions (assert-io-called, assert-no-io, assert-io-count, etc.)

15 harness tests passing on both OCaml (1116/1116) and JS (15/15).
Loaded automatically by both test runners.

MCP tool: sx_harness_eval — evaluate SX with mock IO, returns result + IO trace.

The harness is extensible: new platforms just add entries to the platform dict.
Components can ship with deftest forms that verify IO behavior against mocks.
Tests are independent objects that can be published separately (by CID).

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

hosts/javascript/run_tests.js

@@ -279,6 +279,18 @@ for (const expr of frameworkExprs) {
   Sx.eval(expr, env);
 }
 
+// Load test harness (mock IO platform)
+const harnessPath = path.join(projectDir, "spec", "harness.sx");
+if (fs.existsSync(harnessPath)) {
+  const harnessSrc = fs.readFileSync(harnessPath, "utf8");
+  const harnessExprs = Sx.parse(harnessSrc);
+  for (const expr of harnessExprs) {
+    try { Sx.eval(expr, env); } catch (e) {
+      console.error(`Error loading harness.sx: ${e.message}`);
+    }
+  }
+}
+
 // Load compiler + VM from lib/ when running full tests
 if (fullBuild) {
   const libDir = path.join(projectDir, "lib");