VAD first-guess initialisation and consistency check

[vlouf]

Add VAD (Velocity Azimuth Display) as an optional, confidence-weighted module, in two roles:

1. First guess — reconstruct an expected Vr field from the VAD wind profile to seed reference radials and, in ambiguous gates, decide the number of Nyquist intervals (continuity still sets the precise value).
2. Consistency check — post-pass QC comparing the dealiased field to the VAD-reconstructed field, correcting gates whose residual lands near an integer multiple of 2·V_Nyq.

VAD is derived from the same volume, so this preserves UNRAVEL's no-external-reference property.