feat: add native Export for Snapmaker U1 (bl2u1 integration)

[kbaker827]

Summary

This PR integrates the bl2u1 converter as a native OrcaSlicer feature, adding a File → Export → Export for Snapmaker U1... menu item.

bl2u1 is a standalone Flask web app that converts Bambu Studio .3mf projects to Snapmaker U1-compatible format. This integration brings that conversion directly into OrcaSlicer with no external tools needed.

What it does

• Adds a filament-mapping dialog that shows each extruder slot with a colour picker and filament-type dropdown (max 4, matching U1 hardware)
• Auto-maps filament types → Snapmaker U1 preset names (PLA → Snapmaker PLA SnapSpeed @U1, etc.)
• Applies the Snapmaker U1 printer profile from the profile library to the exported config
• Pads the filament list to exactly 4 slots, filling unused slots with white PLA (same behaviour as bl2u1)
• Remaps per-volume extruder IDs so multi-colour prints transfer correctly
• Exports the modified project non-destructively (live scene is not changed) via the existing store_bbs_3mf() infrastructure

Files changed

File	Change
src/slic3r/GUI/ExportSnapmakerU1.hpp	New – types, constants, ExportSnapmakerU1Dialog class
src/slic3r/GUI/ExportSnapmakerU1.cpp	New – dialog UI implementation
src/slic3r/GUI/Plater_export_snapmaker_u1.cpp	New – Plater::export_snapmaker_u1()
src/slic3r/GUI/Plater.hpp	+1 line – function declaration
src/slic3r/GUI/MainFrame.cpp	+3 lines – menu item after "Export Generic 3MF"
src/slic3r/CMakeLists.txt	+2 lines – register new source files

User flow

File → Export → Export for Snapmaker U1…
        │
        ▼
  Filament mapping dialog
  ┌──────────────────────────────────────┐
  │ ✓  Filament 1  [■ colour]  [PLA ▾]  │
  │    Profile: Snapmaker PLA SnapSpeed  │
  │    → Slot 1                          │
  │ ✓  Filament 2  [■ colour]  [PETG ▾] │
  │    → Slot 2                          │
  │ □  Filament 3  (excluded)            │
  └──────────────────────────────────────┘
        │ OK  →  wxFileDialog  →  .3mf

Test plan

• Build compiles without errors on Linux, macOS, Windows
• Menu item appears under File → Export when a model is loaded
• Menu item is greyed out when no model is loaded
• Dialog shows correct filament count from the active project
• Enabling >4 filaments is blocked with an appropriate message
• Output .3mf opens in OrcaSlicer with Snapmaker U1 printer selected
• Output .3mf opens in Snapmaker Luban without errors
• Multi-colour model retains correct extruder assignments after export
• Live scene is unchanged after export (non-destructive)

🤖 Generated with Claude Code