Update formatting to use 4‑space indentation

- Adjust rustfmt.toml to set tab_spaces = 4.
- Reformat imports, struct definitions, function bodies, and match arms
  across the project to match the new style (aligned fields, added
  spacing, etc.).
- Ensure all code complies with the updated rustfmt settings.

rustfmt.toml

@@ -3,7 +3,7 @@ style_edition = "2024"
 
 max_width = 100
 hard_tabs = false
-tab_spaces = 2
+tab_spaces = 4
 newline_style = "Unix"
 
 reorder_imports = true