Add compatible docking target port selector

[maddavo]

Summary

• add left/right controls to Docking Autopilot for cycling through docking ports on the target vessel
• filter target ports against the vessel's current control-from-here docking port, including node type, multi-type, and gender compatibility
• exclude already docked ports and disable selection while the docking autopilot is active
• update both MechJeb's target controller and KSP's global vessel target so Docking Port Alignment Indicator and other docking aids observe the selection
• show a short status when the target vessel is not yet loaded nearby

Testing

• Release build of MechJeb2.csproj: 0 warnings, 0 errors
• compatibility checks passed for matching/mismatched sizes, multi-type overlap, and gendered ports
• manually tested in KSP 1.12.5 using a stable-compatible MechJeb 2.15.3 build with Docking Port Alignment Indicator 6.13.1; port cycling worked and DPAI followed the selected target port
• existing MechJebLib test suite: 8,383 passed; 3 unrelated failures remained in untouched code (StaticTests.ToSITest, MainSailTinCanVacuum, and MainSailTinCanAtmo)

Screenshots

Target vessel selected, with five compatible ports available:

A compatible target port selected using the new arrow controls: