Antenna-decorrelation measurement harness

[josephnef]

Part of the spatial-diversity roadmap #127.

Goal

Every diversity claim rests on the chains' antennas being decorrelated — on a
small airframe they may not be. This issue builds the harness that measures
decorrelation instead of assuming it, so the rest of the roadmap reports real gain,
not theoretical.

Scope

• Capture per-chain channel estimates (or per-chain RSSI/EVM) under motion / fading
  and compute the envelope correlation between chains.
• Report the realized diversity gain vs the theoretical ~10·log10(N) dB, as a
  function of antenna separation and polarization.
• Provide a repeatable bench + field procedure (scripted, with cleanup) that other
  issues can invoke to qualify a given antenna layout.

Why first

This is the reality check under #C (MRC), #D/#E (TX diversity), and #F (the
localized-corruption argument). A layout with correlated antennas caps all of them.