Add stereo-linked channels, busses, and matrices

[iibaranov-IG]

Issue #8 has remained open while the earlier implementation in #84 drifted away from the current develop architecture. This ports the core linked-channel work to the current Config/CtrlClient split and keeps the change focused on the mixer behavior and UI.

• add a Setup → Mixer Config page for 20 channel/AUX pairs, 8 bus pairs, and 3 matrix pairs
• mirror channel processing and routing parameters across linked pairs
• route consecutive sources to the left/right sides of a new stereo pair
• expose stereo pan and width controls while preserving the current channel level during link/unlink
• collapse linked pairs to one surface strip so they do not consume two fader slots
• mark both sides as [ST L] and [ST R] in the routing table
• append the new mixer-parameter IDs so existing scene IDs remain stable
• prevent client OSC echoes when the mixer returns the mirrored right-side update

The implementation is based on the feature direction from #84 and retains co-author credit for Niklas Arnitz's original work.

Validation:

• Linux PC SDL2 target builds successfully
• doctest: 8 test cases, 63 assertions, all passing

Closes #8