perf: build external cmake deps in parallel

cmake --build was invoked with no --parallel, so the default Unix
Makefiles generator compiled external deps (DPP, msquic, glslang, …)
one translation unit at a time, leaving all but one core idle.

Pass an explicit --parallel N using hardware_concurrency() so dep
builds use the available cores. An explicit count (not a bare
--parallel) avoids an unbounded make -j fork bomb on the Makefiles
generator.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

implementations/Crafter.Build-External.cpp

@@ -210,7 +210,13 @@ std::string ConfigureCMake(const fs::path& cloneDir, const fs::path& cmakeBuildD
 }
 
 std::string BuildCMake(const fs::path& cmakeBuildDir) {
-    std::string cmd = std::format("cmake --build {}", ShellQuote(fs::absolute(cmakeBuildDir).string()));
+    // Without --parallel, the Unix Makefiles generator builds one translation
+    // unit at a time, leaving every core but one idle. Pass an explicit job
+    // count (a bare --parallel maps to an unbounded `make -j` fork bomb on the
+    // Makefiles generator) so deps like DPP/msquic/glslang compile ~N× faster.
+    unsigned jobs = std::max(1u, std::thread::hardware_concurrency());
+    std::string cmd = std::format("cmake --build {} --parallel {}",
+        ShellQuote(fs::absolute(cmakeBuildDir).string()), jobs);
     CommandResult r = RunCommandChecked(cmd);
     if (r.exitCode != 0) {
         return std::format("cmake --build failed (exit {}): {}", r.exitCode, r.output);