Default to auto-detected 64-bit MSBuild on 64-bit hosts (breaking change, v4)

Fixes #88

- msbuild-architecture no longer hardcodes a default of "x86". When the
  input is left unspecified, the action now auto-detects: it prefers x64
  when running on a 64-bit machine and the resolved VS/MSBuild install is
  17.0+ (VS 2022+), and falls back to x86 otherwise. An explicit
  msbuild-architecture value is always respected.
- Extracted resolveMSBuildArchitecture()/parseMajorVersion() as pure,
  exported helpers and added jest tests covering the resolution matrix.
- Bumped package.json version to 4.0.0 (breaking change/new major version).
- Updated action.yml input description and README (usage examples now
  reference @v4, new "Breaking Changes in v4" section, and the
  architecture-selection docs rewritten to describe auto-detection).
- Rebuilt dist/index.js via ncc.

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

dist/index.js

@@ -26,13 +26,45 @@ Object.defineProperty(exports, "__esModule", ({ value: true }));
 const core = __importStar(__nccwpck_require__(484));
 const exec = __importStar(__nccwpck_require__(236));
 const fs = __importStar(__nccwpck_require__(896));
+const os = __importStar(__nccwpck_require__(857));
 const path = __importStar(__nccwpck_require__(928));
 const io = __importStar(__nccwpck_require__(994));
 const IS_WINDOWS = process.platform === 'win32';
 const VS_VERSION = core.getInput('vs-version') || 'latest';
 const VSWHERE_PATH = core.getInput('vswhere-path');
 const ALLOW_PRERELEASE = core.getInput('vs-prerelease') || 'false';
-let MSBUILD_ARCH = core.getInput('msbuild-architecture') || 'x86';
+const MSBUILD_ARCH_INPUT = core.getInput('msbuild-architecture');
+let MSBUILD_ARCH = MSBUILD_ARCH_INPUT;
+// The minimum Visual Studio/MSBuild major version that ships a native x64 MSBuild.exe
+exports.MIN_VS_VERSION_FOR_X64 = 17;
+// Determines the effective MSBuild architecture to use.
+// If the user explicitly specified `msbuild-architecture`, that choice is always respected.
+// Otherwise (breaking change as of v4), the action auto-detects and prefers x64 when
+// running on a 64-bit OS and the resolved VS/MSBuild installation is version 17.0 or later.
+// If the VS version can't be determined, or the host isn't 64-bit, it falls back to x86.
+function resolveMSBuildArchitecture(explicitArch, isWindows64Bit, vsMajorVersion) {
+    if (explicitArch) {
+        return explicitArch;
+    }
+    if (isWindows64Bit &&
+        vsMajorVersion !== undefined &&
+        vsMajorVersion >= exports.MIN_VS_VERSION_FOR_X64) {
+        return 'x64';
+    }
+    return 'x86';
+}
+exports.resolveMSBuildArchitecture = resolveMSBuildArchitecture;
+// Parses the major version number out of a vswhere `installationVersion` string
+// such as "17.9.34728.123". Returns undefined if it cannot be parsed.
+function parseMajorVersion(installationVersion) {
+    const match = installationVersion.trim().match(/^(\d+)/);
+    if (!match) {
+        return undefined;
+    }
+    const major = parseInt(match[1], 10);
+    return isNaN(major) ? undefined : major;
+}
+exports.parseMajorVersion = parseMajorVersion;
 // if a specific version of VS is requested
 let VSWHERE_EXEC = '-products * -requires Microsoft.Component.MSBuild -property installationPath -latest ';
 if (ALLOW_PRERELEASE === 'true') {
@@ -75,6 +107,27 @@ function run() {
                 return;
             }
             core.debug(`Full tool exe: ${vswhereToolExe}`);
+            // if the user did not explicitly specify an architecture, auto-detect the
+            // preferred architecture based on the host OS and the resolved VS/MSBuild version
+            if (!MSBUILD_ARCH_INPUT) {
+                let vsMajorVersion;
+                const versionOptions = {};
+                versionOptions.listeners = {
+                    stdout: (data) => {
+                        const installationVersion = data.toString().trim();
+                        core.debug(`Found installation version: ${installationVersion}`);
+                        const parsed = parseMajorVersion(installationVersion);
+                        if (parsed !== undefined) {
+                            vsMajorVersion = parsed;
+                        }
+                    }
+                };
+                const versionExec = VSWHERE_EXEC.replace('-property installationPath', '-property installationVersion');
+                yield exec.exec(`"${vswhereToolExe}" ${versionExec}`, [], versionOptions);
+                const isWindows64Bit = os.arch() === 'x64' || os.arch() === 'arm64';
+                MSBUILD_ARCH = resolveMSBuildArchitecture(MSBUILD_ARCH_INPUT, isWindows64Bit, vsMajorVersion);
+                core.info(`Auto-detected msbuild-architecture: ${MSBUILD_ARCH} (os.arch=${os.arch()}, vsMajorVersion=${vsMajorVersion})`);
+            }
             let foundToolPath = '';
             const options = {};
             options.listeners = {