Fix dlx-binary Windows shell execution by adding cache dir to PATH

When spawning .cmd/.bat files on Windows with shell: true, cmd.exe uses PATH
+ PATHEXT to locate executables. Since dlx downloads to a custom cache
directory not in PATH (unlike system package managers), we must prepend the
cache directory to PATH so cmd.exe can find the binary after spawn strips it
to basename.

Author: jdalton

registry/src/lib/dlx-binary.ts

@@ -294,12 +294,35 @@ export async function dlxBinary(
   }
 
   // Execute the binary.
-  // On Windows, script files (.bat, .cmd, .ps1) require shell: true.
+  // On Windows, script files (.bat, .cmd, .ps1) require shell: true because
+  // they are not executable on their own and must be run through cmd.exe.
   // Note: .exe files are actual binaries and don't need shell mode.
-  const finalSpawnOptions =
-    WIN32 && /\.(?:bat|cmd|ps1)$/i.test(binaryPath)
-      ? { ...spawnOptions, shell: true }
-      : spawnOptions
+  const needsShell = WIN32 && /\.(?:bat|cmd|ps1)$/i.test(binaryPath)
+  // Windows cmd.exe PATH resolution behavior:
+  // When shell: true on Windows with .cmd/.bat/.ps1 files, spawn will automatically
+  // strip the full path down to just the basename without extension (e.g.,
+  // C:\cache\test.cmd becomes just "test"). Windows cmd.exe then searches for "test"
+  // in directories listed in PATH, trying each extension from PATHEXT environment
+  // variable (.COM, .EXE, .BAT, .CMD, etc.) until it finds a match.
+  //
+  // Since our binaries are downloaded to a custom cache directory that's not in PATH
+  // (unlike system package managers like npm/pnpm/yarn which are already in PATH),
+  // we must prepend the cache directory to PATH so cmd.exe can locate the binary.
+  //
+  // This approach is consistent with how other tools handle Windows command execution:
+  // - npm's promise-spawn: uses which.sync() to find commands in PATH
+  // - cross-spawn: spawns cmd.exe with escaped arguments
+  // - Node.js spawn with shell: true: delegates to cmd.exe which uses PATH
+  const finalSpawnOptions = needsShell
+    ? {
+        ...spawnOptions,
+        env: {
+          ...spawnOptions?.env,
+          PATH: `${cacheEntryDir}${path.delimiter}${process.env['PATH'] || ''}`,
+        },
+        shell: true,
+      }
+    : spawnOptions
   const spawnPromise = spawn(binaryPath, args, finalSpawnOptions, spawnExtra)
 
   return {