ci: add a real-MSVC (cl.exe/nmake) build job

Add a build-msvc job on windows-latest that uses
step-security/msvc-dev-cmd to set up the MSVC developer environment,
builds NASM with 'nmake /f Mkfiles/msvc.mak', and smoke-tests the
resulting nasm.exe/ndisasm.exe by assembling and disassembling a tiny
program and building a win64 object file.

step-security/msvc-dev-cmd is a StepSecurity-maintained, drop-in
replacement for the more widely known ilammy/msvc-dev-cmd@v1 action;
the latter's newest release (v1.13.0, Jan 2024) still targets the
now-deprecated Node.js 20 runtime with no newer release in sight,
while step-security's fork tracks Node.js 24.

This is the first CI coverage of the MSVC build path (Mkfiles/msvc.mak
+ config/msvc.h), which previously had no automated verification.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/test.yml

@@ -43,3 +43,36 @@ jobs:
           name: travis-log
           path: travis.log
           if-no-files-found: warn
+
+  build-msvc:
+    name: Build with MSVC (Windows)
+    runs-on: windows-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Set up MSVC developer environment
+        # step-security/msvc-dev-cmd is a StepSecurity-maintained,
+        # actively updated (Node 24) drop-in replacement for
+        # ilammy/msvc-dev-cmd@v1, which is Node 20 and has no newer
+        # release addressing that.
+        uses: step-security/msvc-dev-cmd@v1
+        with:
+          arch: x64
+
+      - name: Build with nmake/cl.exe
+        shell: cmd
+        run: |
+          nmake /f Mkfiles\msvc.mak
+
+      - name: Smoke-test the resulting binaries
+        shell: cmd
+        run: |
+          nasm.exe --version
+          ndisasm.exe --version
+          echo mov eax, 1 > smoke.asm
+          echo ret >> smoke.asm
+          nasm.exe -f bin smoke.asm -o smoke.bin
+          ndisasm.exe -b 64 smoke.bin
+          nasm.exe -f win64 smoke.asm -o smoke.obj