Move Xcode project and example under MathEditor directory

[maitbayev]

Motivation

• Reorganize the repository to group the Xcode project and example app under the MathEditor package directory for clearer structure and packaging.
• Keep project layout consistent with the Swift package in MathEditor/ so related artifacts are colocated.

Description

• Moved the top-level MathEditor.xcodeproj into MathEditor/MathEditor.xcodeproj as a path-only rename with no content edits.
• Moved the MathEditorExample/ app folder into MathEditor/MathEditorExample/ as a path-only rename with no content edits.
• All files were preserved exactly; changes are pure renames across the project/workspace and example resource files.
• No source code or project file contents were modified during the move.

Testing

• No automated test suite was executed because this change is a path-only rename that does not modify file contents.
• Repository integrity and file presence were validated by listing and opening the moved files to confirm they exist in their new locations and are unchanged.

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Codex Task