feat(vpm): compute in-plane hub force (H-force) in the free-wake march

[mcroomp]

Summary

Follow-up to #10. The BEM-family models (QS BEM, Pitt-Peters, Oye) now report the in-plane hub force ("H-force"), but the VPM free-wake rotor did not: it accumulated only the out-of-plane thrust and hub moments, then passed 0.0, 0.0 for fx_hub/fy_hub into assemble_result, so F_world carried no in-plane force for VPM.

This PR closes that gap by accumulating the H-force directly in the VPM time-domain loads loop.

What changed

• dynbem_rs/src/vpm/march.rs — per element, accumulate:

  • fx += dF_t*sin(psi) - dT*sin(beta_b)*cos(psi)
  • fy += dF_t*cos(psi) + dT*sin(beta_b)*sin(psi)

  The first term is the profile/induced-drag H-force (identical pairing to bem_common::SweepCtx::run). The second is the flapping-tilt term, resolved instantaneously from the real per-blade flap DOF beta_b — no b1c/b1s harmonics, and exactly zero for a rigid rotor. Unlike the BEM-family models the VPM needs no apply_flap_dynamics: flapback emerges from the marched disk geometry.

• dynbem_rs/src/vpm/mod.rs — thread fx_hub/fy_hub through VpmRotorResult and pass them from step() into assemble_result (removed the TODO).

• validation_rs — new vpm_hforce_directional check: downwind H-force in edgewise flow, ~0 in hover, mostly aligned with the flow axis, larger than the hover baseline. Registered in checks/mod.rs; fixed the VpmRotorResult initializer in cyclic_phase_servo.rs.

• Docs — VPM_DESIGN.md §5.5.5 (hub-load equations) + §8.1 validation table; AGENTS.md new "VPM in-plane H-force" subsection documenting the sign convention.

Sign note

FlightCondition::v_hub is the freestream air velocity in the hub frame (relative wind = v_hub − blade velocity), so a +X freestream yields a downwind +X in-plane force — matching the level-disk crosswind direction validated in checks/h_force.rs.

Validation

• Full validation_rs suite: 166/166 pass (was 162, +4 new VPM H-force assertions).
• cargo test --release -p dynbem_rs -- vpm: 18 pass.
• cargo fmt clean; dynbem PyO3 crate builds.

Note: the untracked steady_flight_simple/ directory is intentionally excluded as unrelated.