Add Mouse I/O peripheral with tests and example

[KuznetsovNick]

Adds a memory-mapped Mouse peripheral, available from the I/O panel
alongside the existing LED Matrix / Switches / D-Pad.

Behavior

The widget reports cursor position, button state and scroll wheel of the
mouse over its drawing area. The reported coordinate range is
configurable via the Width and Height parameters.

Memory map

Name	Offset	Size	R/W	Description
X	0x00	32	R	Cursor X within [0, Width)
Y	0x04	32	R	Cursor Y within [0, Height)
LBUTTON	0x08	1	R	1 while the left mouse button is held
RBUTTON	0x0C	1	R	1 while the right mouse button is held
SCROLL	0x10	32	R/W	Accumulated wheel ticks (signed); writeable to reset/seed

Width and Height are also exposed as the assembler symbols
MOUSE_<n>_WIDTH / MOUSE_<n>_HEIGHT.

Example

examples/C/mouseDrawing.c — hold LMB to draw, RMB to erase a small
area, wheel to cycle the color (R → G → B). Coordinates are scaled and
clamped to the LED Matrix size, so the demo works for any combination
of mouse and matrix dimensions.

Tests

test/tst_io_mouse.cpp — covers register layout, values across
synthesized cursor/button/wheel events, and reset.