Stub out previous undo implementation; update docs.

- Remove outdated `undo-state.md`
- Add two code quality/optimization reports that were used to guide previous work:
  - `code-report.md` (optimization)
  - `code-report-quality.md` (stability and code health)
- Add `themes.md`.
- Update undo system docs and roadmap.

.idea/codeStyles/Project.xml

@@ -141,6 +141,9 @@
         <pair source="c++m" header="" fileNamingConvention="NONE" />
       </extensions>
     </files>
+    <codeStyleSettings language="Markdown">
+      <option name="RIGHT_MARGIN" value="72" />
+    </codeStyleSettings>
     <codeStyleSettings language="ObjectiveC">
       <indentOptions>
         <option name="INDENT_SIZE" value="8" />