Spatial × frequency — space-frequency interleave for the outer FEC

[josephnef]

Part of the spatial-diversity roadmap #127.

Goal

Combine the spatial and frequency diversity axes into a space-frequency
interleave for the outer FEC. The two axes are orthogonal (one LO per chip: the
chains diversify within a channel, hops diversify across channels), so a symbol can
be indexed on both.

The idea

The outer code today spreads symbol i onto hop channel i mod N_ch
(docs/frequency-hopping.md), converting a single-channel outage into a
recoverable erasure. Adding the spatial axis, a symbol is placed on a
(channel, spatial-diversity-mode) coordinate, so an erasure requires a fade that is
both narrowband and spatially deep — a strictly rarer event.

Scope

• Extend the frequency-diversity recovery simulator to a 2-D (channel x space)
  interleave and derive the recovery threshold vs the 1-D hop-only case.
• Model correlated vs independent space/frequency fades (the pessimistic case where
  the two axes are not independent).
• Define the interleave/combine rule so the existing dedup-by-(block,symbol)
  erasure decoder still just adds up feeds.