Add an upright differential input to MathKeyboard

[charlesroddie]

Add MathKeyboardInput.Differential alongside PartialDifferential. Pressing it should insert an upright differential d represented as \mathrm{d}, not the italic variable d.

Acceptance criteria:

• Add a distinct public MathKeyboardInput.Differential value.
• MathKeyboard.KeyPress inserts an upright d atom and leaves the caret after it.
• Serialization produces \mathrm{d} consistently.
• Add keypress tests for the new input alone and in a representative expression such as \int f(x),\mathrm{d}x.
• Expose the input through the sample/platform keyboard controls that enumerate MathKeyboardInput values.