Support subtype INTERNAL fields

[Sean-Kenneth-Doherty]

Summary

• let each selected B9 subtype provide its own nested INTERNAL values for Reviva's runtime part config
• keep the runtime INTERNAL name synchronized with the selected subtype
• document how subtype authors can provide fields such as offset for part/internal relationships that need them

Addresses #35.

Context

Issue #35 is about IVA switches that need per-part/per-internal fields such as offset. This implementation does not copy one shared part INTERNAL node across every selected IVA. Instead, Reviva creates a fresh runtime INTERNAL node for the selected subtype, sets that subtype's internal name, and copies only extra fields from an INTERNAL node nested under that same subtype's ModuleIVASwitch data.

Validation

• git diff --check
• dotnet build Reviva.sln -c Release --no-restore /p:KSPRoot="/home/sean/.local/share/Steam/steamapps/common/Kerbal Space Program": 0 warnings, 0 errors
• KSP 1.12.5 main-menu runtime harness with ModuleManager 4.2.3 and B9PartSwitch 2.21.0.4:
  • baseline Reviva reached methodSignature=newName and failed to copy subtype offset/scale
  • patched branch reached methodSignature=newName,moduleConfig and passed with afterOffset="1.25, -2.5, 3.75" and afterScale="0.95, 0.95, 0.95"

Caveat

This remains a draft because the runtime proof is a targeted headless KSP harness against the loaded part prefab/config path, not a real flight/B9 IVA switch visual smoke test in the shuttle scenario from the issue.