[nexus] add WebAssembly support using Emscripten (#12904)

This commit adds support for building the Nexus simulator for
WebAssembly (WASM) using the Emscripten toolchain. This enables the
simulator to run in a web browser environment with a JavaScript-based
control interface and visualization.

Key implementation details:
- Introduced `nexus_wasm.cpp` which defines Emscripten bindings (using
  Embind) for core simulation controls, including stepping time,
  node creation, topology orchestration, and state manipulation.
- Implemented a `WasmObserver` and a global event queue to capture
  simulation events (node state changes, link updates, packet events)
  and expose them to JavaScript via a polling mechanism (`pollEvent`).
- Updated the CMake build system to support the `EMSCRIPTEN` platform,
  configuring specific linker options for ES6 module export,
  modularization, and memory growth.
- Enhanced `build.sh` to allow targeting WASM via `emcmake`.
- Guarded file-system-dependent operations in `nexus_pcap.cpp` and
  adjusted `nexus_core.cpp` to handle WASM-specific constraints where
  standard I/O or multiple observers might not be applicable.
- Added `test_wasm_bindings.mjs`, a Node.js-based smoke test that
  verifies the integrity of the WASM bindings and event pipeline.
- Integrated `nexus-wasm-tests` into the GitHub Actions workflow to
  ensure continuous verification of the WASM build and functionality.

.github/workflows/nexus.yml

@@ -133,3 +133,33 @@ jobs:
     - name: Run GRPC Tests
       run: |
         cd build/nexus && ./tests/nexus/nexus_grpc
+
+  nexus-wasm-tests:
+    name: nexus-wasm-tests
+    runs-on: ubuntu-24.04
+    steps:
+    - name: Harden Runner
+      uses: step-security/harden-runner@5ef0c079ce82195b2a36a210272d6b661572d83e # v2.14.2
+      with:
+        egress-policy: audit
+
+    - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      with:
+        submodules: recursive
+
+    - name: Bootstrap
+      run: |
+        sudo apt-get update
+        sudo apt-get --no-install-recommends install -y ninja-build
+
+    - name: Set up Emscripten
+      uses: mymindstorm/setup-emsdk@v14
+
+    - name: Build Nexus WASM
+      run: |
+        mkdir -p build/nexus
+        top_builddir=build/nexus ./tests/nexus/build.sh wasm
+
+    - name: Run WASM Smoke Test
+      run: |
+        node tests/nexus/test_wasm_bindings.mjs build/nexus/tests/nexus/nexus_live_demo.js