add fs lib to walk file tree

fs.go

@@ -0,0 +1,59 @@
+/*
+This file has the raw file io operations that are used by FsTree
+or elsewhere in the app. Nowhere else should there be a direct
+fileopen
+
+- ruinivist, 30Dec25
+*/
+
+package main
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+)
+
+func walkFileSystemAndBuildTree(rootPath string, node *FsNode) error {
+	if node == nil {
+		return errors.New("node cannot be nil")
+	}
+	if len(node.children) > 0 {
+		return errors.New("node already has children")
+	}
+
+	entries, err := os.ReadDir(rootPath)
+	if err != nil {
+		return err
+	}
+
+	for _, entry := range entries {
+		childPath := filepath.Join(rootPath, entry.Name())
+
+		var nodeType FsNodeType
+		if entry.IsDir() {
+			nodeType = FolderNode
+		} else {
+			nodeType = FileNode
+		}
+
+		childNode := &FsNode{
+			nodeType: nodeType,
+			path:     childPath,
+			children: make([]*FsNode, 0),
+			parent:   node,
+			expanded: false,
+		}
+
+		node.children = append(node.children, childNode)
+
+		// Recursively walk subdirectories
+		if entry.IsDir() {
+			if err := walkFileSystemAndBuildTree(childPath, childNode); err != nil {
+				return err
+			}
+		}
+	}
+
+	return nil
+}