Add Seven Segment I/O peripheral with tests and example

[KuznetsovNick]

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

Behavior

The widget renders N digits, where N is configurable via the # Digits
parameter (1..16, default 4). Color theme is selectable via the Color
parameter. Each digit is driven by a single 8-bit segment mask written
from the program.

Memory map

One 32-bit register per digit, byte offset = digit_index * 4.
Only the low byte is used; bits map to segments as follows:

Bit	7	6	5	4	3	2	1	0
Seg	DP	G	F	E	D	C	B	A

Aligned 32-bit reads return the last value written. Misaligned or
out-of-range accesses return 0 / are ignored.

Example

examples/C/sevenSegmentRunner.c — slides "67" across the display.

Tests

test/tst_io_7indicator.cpp — covers the write/read round trip,
low-byte truncation, alignment handling, out-of-range access, reset,
and resizing via the # Digits parameter.