RX-side in-chip MRC on an 8814 ground station

[josephnef]

Part of the spatial-diversity roadmap #127. Depends on #A (per-chain capability).

Goal

Exploit a 4T4R chip (8814) as a ground receiver doing in-chip
maximal-ratio combining across its RF chains — the single biggest USB-free win in
the roadmap. One 4-chain adapter partially replaces wfb-ng's multi-adapter
diversity array, entirely in-chip, adding zero USB load (still one stream out).

Scope

• Confirm the chip performs MRC across active RX chains and quantify the SNR gain
  vs a single chain, at matched operating point.
• Compare against the wfb-ng multi-adapter diversity baseline (several separate
  dongles, combined in software) for delivered-block rate at equal antenna count.
• Feed the cleaner combined signal into the fused-FEC RX path and measure the
  effect on corrupt-frame rate and sub-block survival (links to #F).

Reality check

Gains are bounded by antenna decorrelation — qualify the layout with #B first.