Close STBC on-air — resolve the 2-channel-alignment root cause

[josephnef]

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

Goal

Close the on-air STBC path so devourer gains a diversity-order-2 transmit
knob (one stream, full spatial diversity, same MCS). STBC currently lives in the
radiotap path but does not produce correct output on air — the open root cause is
a 2-channel alignment problem.

Scope

• Resolve the 2-channel-alignment root cause behind the current on-air STBC failure.
• Validate diversity order 2 on-air with an SDR: fade-margin / outage improvement
  vs single-chain and vs CSD (#D) at matched throughput.
• On the 8814, explore extending beyond the 2x1 Alamouti case where the extra
  chains allow it.

Relation to #D

CSD and STBC are complementary transmit-diversity knobs; this issue adds the
higher-diversity-order option once the CSD knob (#D) is in place.