wboayue / stable-flight / 38612077553

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Sim: tailsitter model and nose-up hover (#349) (#386)

Adds `SimTailsitter` to `stable-flight-sim`: ArduPilot's
`plane-tailsitter` (the default plane with 1.5× thrust,
`SIM_Plane.cpp:78-81`) on two motors and elevons
(`TableMixer::tailsitter_2p`). It takes off from its tail and holds a
nose-up hover on `TailsitterGuidance`/`TailsitterStabilizer`. First of
two PRs for #349; transitions come next.

Refs #349

## Model
- `SimTailsitter` + `TailsitterModel { wing: FixedWingModel,
motor_arm_m, prop_wash: PropWash }`. `SimPlane` and the tailsitter share
one crate-private `FixedWing` dynamics (`wing.rs`); `FixedWingModel` and
`SimPlane` are unchanged in shape.
- **Differential thrust:** each motor thrusts along body x. A mixer yaw
factor acts through `motor_arm_m`.
- **Prop wash:** uses momentum theory along the thrust axis.
- Nose first, the surface terms see `surface_fraction · thrust / disc
area` on top of the free stream, so the elevons have authority in a
hover.
- Sinking tail first, they see the static jet less the sink, so
authority fades with the sink (none at the jet's speed). This is what
the eXplora showed in RealFlight (#123), and it matters for the back
transition (#343).
- ArduPilot instead adds 20 m/s × throttle to the airspeed of every
moment (`SIM_Plane.cpp:154-162`), whatever the direction.
- **Fixed-wing dynamics:** `FixedWing` takes the rotor layout
(`Rotors::airplane`/`Rotors::tailsitter`: arm, wash) and the rest
attitude separately. Surface terms are their own `aero::surface_wrench`.
- **Resting on the tail:** ground contact holds the body at a rest
attitude with its heading kept (ArduPilot `SIM_Aircraft.cpp:745-765`).
The rest attitude is `View::tailsitter`'s rotation undone, so a nose-up
hover reads level in the view. The copter's rest is identity.
- **Aero bounded at any α (affects both fixed-wing vehicles):**
- Drag blends with the lift sigmoid into a flat plate's `c_drag_0 +
2·sin²α`. Before, broadside drag was ≈6 and reversed-flow... (continued)

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