FBP Go: D2 fuel type

**As a** *Fire Behaviour Specialist* **I want** *the D2 fuel type* **So That** *I can predict fire behaviour for green aspen as one can in the red book*

**Additional Context**
- emulating what is done for the m1/m2 when you switch to green mixed wood.
- emulating for aspen with a bui threshold.
- D2 Fuel Type (ros 20% of d1) (below bui threshold there is no ROS, above threshold it's 20% of d1)

**Acceptance Criteria**
- [ ] Add D2 fuel type in dropdown
- [ ] Add math for D2 fuel type

**Definition of Done**
- Ready to Demo in Sprint Review
- Does what I have made have appropriate test coverage?
- Documentation and/or scientific documentation exists and can be found
- Peer Reviewed by 2 people on the team
- Manual testing of all PRs in Dev and Prod
- Merged

Start by locating the existing m1/m2 green mixed wood implementation and the fuel-type dropdown. Review how BUI thresholds and rate of spread are represented, then compare the D2 requirements with the red book specification. Done means D2 is selectable, applies the stated threshold and ROS behavior, and has appropriate tests and scientific documentation.

Written by the indexing model from the issue text.