Fix Windows CI C1060 by reducing build parallelism

Windows CI was hitting C1060 "compiler is out of heap space" even after
splitting test files to 1,200 lines. Root cause: GitHub's windows-latest
runners have only 16GB RAM, and parallel cl.exe processes exhaust memory
with template-heavy code.

Solution (based on Microsoft docs + community solutions):
1. Add `-A x64` to ensure 64-bit toolset (32-bit has ~2-3GB limit)
2. Use `/m:4` to limit MSBuild to 4 worker processes
3. Use `/p:CL_MPCount=1` to disable cl.exe's internal /MP flag

This reduces peak compiler memory usage while still maintaining reasonable
build performance (~4x parallelism at MSBuild level).

Refs:
- https://learn.microsoft.com/en-us/cpp/error-messages/compiler-errors-1/fatal-error-c1060
- appveyor/ci#742

Author: 0xeb

.github/workflows/ci.yml

@@ -51,7 +51,7 @@ jobs:
         if: runner.os == 'Windows'
         run: >
           cmake -B build -S .
-          -G "Visual Studio 17 2022"
+          -G "Visual Studio 17 2022" -A x64
           -DFASTMCPP_BUILD_TESTS=ON
           -DFASTMCPP_BUILD_EXAMPLES=ON
 
@@ -61,8 +61,9 @@ jobs:
 
       - name: Build (Windows)
         if: runner.os == 'Windows'
-        # Single-threaded build on Windows to avoid compiler heap space issues with large test files
-        run: cmake --build build --config ${{ matrix.build_type }} --parallel 1
+        # Reduce parallelism to avoid C1060 "compiler out of heap space" on GitHub's 16GB Windows runners
+        # /m:4 limits MSBuild workers, /p:CL_MPCount=1 disables cl.exe's /MP flag
+        run: cmake --build build --config ${{ matrix.build_type }} -- /m:4 /p:CL_MPCount=1
 
       - name: Test
         run: ctest --test-dir build -C ${{ matrix.build_type }} --output-on-failure