build(native): enable per-TU parallel compilation (cmake --parallel + /MP)

Before: MSBuild defaulted to a single process per project, so each
vcxproj compiled its .cpp files sequentially. Local clean rebuild of
OpenZen.dll + OpenZenLoader.exe measured ~14.6 s.

After: same cold rebuild measures ~4.8 s — 3x faster — by combining

- `cmake --build --parallel <Runtime.runtime.availableProcessors()>`
  in the buildNative task, which forwards a project-level `-m:N` to
  MSBuild and gets DLL / Loader to build concurrently where the
  dependency graph allows.
- `/MP` on the MSVC compile options, which is the actual win: it lets
  cl.exe spawn one compiler process per available core within a
  single vcxproj. The DLL+Loader together have ~13 .cpp files; this
  is where the bulk of wall-time was being spent serially.

CI runners get the same speed-up for free.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

build.gradle

@@ -405,7 +405,13 @@ tasks.register('buildNative', Exec) {
     doFirst {
         def cmake = findCmake()
         if (!cmake) throw new GradleException(cmakeMissingMessage)
-        commandLine cmake, '--build', 'build', '--config', 'Release'
+        // --parallel without a number lets CMake pick the host's logical CPU
+        // count and forwards it to the underlying generator (MSBuild gets /m,
+        // Ninja gets -j). MSBuild defaults to a single process otherwise.
+        def jobs = Runtime.runtime.availableProcessors()
+        logger.lifecycle("Parallel jobs: ${jobs}")
+        commandLine cmake, '--build', 'build', '--config', 'Release',
+                '--parallel', jobs.toString()
     }
 }
 

native/CMakeLists.txt

@@ -37,7 +37,12 @@ add_compile_definitions(
 )
 
 if(MSVC)
-    add_compile_options(/W3 /permissive- /utf-8 /EHsc)
+    # /MP enables cl.exe-level multi-process compilation: every translation
+    # unit inside a single vcxproj compiles in parallel. Without it, MSBuild
+    # only parallelises *across* projects (the -j N forwarded by
+    # `cmake --build --parallel` controls that), so a single project with
+    # a dozen .cpp files would still compile serially.
+    add_compile_options(/W3 /permissive- /utf-8 /EHsc /MP)
 endif()
 
 add_subdirectory(dll)