TX cyclic-shift diversity (CSD) as a validated diversity knob

[josephnef]

Part of the spatial-diversity roadmap #127. Depends on #A.

Goal

Promote cyclic shift diversity from a validated primitive to a first-class
robustness knob on the TX (drone) side. CSD sprays the single video stream
across 2–4 chains with cyclic shifts — flat-fade transmit diversity at no rate
cost and no extra USB load. It is the lowest-risk entry point to TX spatial
diversity (STBC on-air is still unresolved; CSD is chip-validated).

Scope

• Expose CSD as a selectable per-frame mode (like rate/BW/STBC in the radiotap path)
  so it can be driven per-packet and per-layer.
• Measure the fade-margin improvement on-air (SDR ground truth) vs single-chain TX
  across 2 and (on 8814) 3–4 chains.
• Characterize the energy cost (extra chains' baseline draw) as input to #H.

Reality check

Diversity gain bounded by antenna decorrelation (#B).