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/javascript/run_tests.js

@@ -306,6 +306,31 @@ if (fullBuild) {
       }
     }
   }
+  // Load web harnesses (DOM mocking, signals, rendering awareness)
+  const webDir = path.join(projectDir, "web");
+  for (const webFile of ["harness-web.sx", "harness-reactive.sx"]) {
+    const wp = path.join(webDir, webFile);
+    if (fs.existsSync(wp)) {
+      const src = fs.readFileSync(wp, "utf8");
+      const exprs = Sx.parse(src);
+      for (const expr of exprs) {
+        try { Sx.eval(expr, env); } catch (e) {
+          console.error(`Error loading ${webFile}: ${e.message}`);
+        }
+      }
+    }
+  }
+  // Load stepper-lib (shared stepper functions used by lib/tests/test-stepper.sx)
+  const stepperLibPath = path.join(projectDir, "sx", "sx", "stepper-lib.sx");
+  if (fs.existsSync(stepperLibPath)) {
+    const src = fs.readFileSync(stepperLibPath, "utf8");
+    const exprs = Sx.parse(src);
+    for (const expr of exprs) {
+      try { Sx.eval(expr, env); } catch (e) {
+        console.error(`Error loading stepper-lib.sx: ${e.message}`);
+      }
+    }
+  }
 }
 
 // Determine which tests to run