Instrument backlighting with emissive dimmer

[vanlueckn]

Summary

• New motion type Motion::Emissive: scales a material's emissive colour by a value (0…1) instead of moving the node, enabling smooth dimming of backlighting
• Cab input CabInputs::instrument_light (f64, 0…1) with CabControl::InstrumentLight for keyboard control (,/. keys) and interactive 3D cab binding
• UI system in models.rs:
  • animate_backlight() system updates emissive colour of glowing parts per frame
  • GlowMesh component tracks base emissive colour and parent part node
  • Material cloning during bind to preserve per-part emissive scaling
• BR 101 example model includes instrument_light node with emissive material, demonstrating content integration
• Editors updated: vehicle editor, signal editor UI to support the new motion type
• Documentation in MODS.md and STATUS.md covering usage pattern and technical notes
• i18n strings added to both en and de locales (motion-glow, cab-input-instrument-light, hud-keys-lights)

Testing

• Verified i18n keys exist in both crates/i18n/locales/en/main.ftl and crates/i18n/locales/de/main.ftl
• Confirmed Motion::Emissive integrated into all editor motion-selection paths (vehicle, signal, UI)
• BR 101 model includes functional instrument_light binding with Motion::Emissive
• Keyboard input (, and .) integrated into player_input() system for continuous dimmer control
• HUD updated to display lights control keys under separate hud-keys-lights line
• All part_value() branches return appropriate values for the new switch:instrument_light and switch:cab_light functions
• Not run: full runtime test of dimming interaction with the 3D cab and material emissive scaling