Per-temporal-layer spatial UEP (diversity order as a per-layer knob)

[josephnef]

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

Goal

Add spatial diversity order as a per-temporal-layer protection knob, extending
the existing unequal-error-protection design (docs/adaptive-link.md,
docs/fused-fec.md), which today varies protection across layers on two knobs
(modulation + FEC rate). This adds a third.

The idea

Base / IDR layers fly with transmit diversity (STBC / more chains) for the deepest
fade protection; enhancement layers fly single-chain and cheap. Protection then
tracks importance on three axes at once — modulation, FEC rate, and spatial
diversity order — producing a finer graceful-degradation staircase and folding the
spatial energy cost (#H) into the per-layer cost function.

Scope

• Define the per-layer spatial ladder (which layers get which diversity mode) and
  integrate it with the existing MCS + FEC-rate ladders.
• Simulate the three-axis staircase under a fly-out/back path-loss schedule and show
  base-layer delivery holding while enhancement sheds.
• Quantify the energy cost of per-layer diversity via the #H model.