feat: add native Windows hierarchy test harness

internal/config/path.go

@@ -3,6 +3,7 @@ package config
 import (
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -21,10 +22,13 @@ func validateAbsolutePath(name, value string, platform Platform, allowEmpty bool
 		}
 		return cleanWindowsPath(value), nil
 	}
-	if !filepath.IsAbs(value) {
+	// Config parsing can validate an inactive Unix shell path while running on
+	// Windows. filepath follows the host OS, whereas the config field's stated
+	// platform is Unix, so use slash-path semantics here.
+	if !path.IsAbs(value) {
 		return "", fmt.Errorf("%s must be an absolute Unix path", name)
 	}
-	return filepath.Clean(value), nil
+	return path.Clean(value), nil
 }
 
 func isWindowsAbsolute(value string) bool {