Link-probe MCS-headroom axis, thermal overlay, rendezvous beacon

[josephnef]

Completes the active link-probe and adds the remaining adaptive-link building-block
harnesses — follow-up to #178.

MCS-headroom axis

DEVOURER_TX_MCS_SWEEP="MCS0,MCS2,…" steps the on-air rate through a list (a
beacon-feed rate sweep, decoupled from the idle-hold continuous carrier like the
power ramp), marking each step with <devourer-contx>mcs=. link_probe.py
auto-detects the swept axis, aggregates per rate across sweep cycles, and for the
MCS axis recommends the highest rate whose ground SNR clears the target — the
"ride the fastest modulation the link holds" reflex. link_probe.sh gains
--axis power|mcs. (The prior version claimed the harness served the MCS case but
only implemented the power axis — this makes it real.)

Thermal-budget overlay

The probe polls the emitter's PA thermal meter during the sweep and reports the
drift, bounding the sustainable power/duty — the drone's local safety input.

Rendezvous beacon

tests/rendezvous.sh: the ground parks a modulated continuous beacon on a
channel; the drone scans candidate channels with its frame-free energy sensor and
locks onto the beacon's channel — the asymmetric-duty rendezvous composed from the
continuous-TX stimulus and the RX energy sensor.

Docs

Note that the continuous carrier is a spectral/thermal stimulus, not a clean
frame source (its looped payload isn't FCS-valid), which is why the probe uses a
beacon feed for decodable per-frame SNR; and flag the 100%-duty PA heat as a
debug/characterisation caveat.

Testing

• Build clean; ctest green (4/4).
• Hardware-validated (two adapters, no SDR):
  • MCS axis: per-rate SNR curve + correct "highest rate that holds the floor"
    recommendation.
  • Power axis: unchanged (monotonic curve + "cheapest power" recommendation).
  • Rendezvous: the scanner locked onto the beacon's channel (energy ~27× floor).
  • Decodability characterised: the Jaguar1 continuous carrier is PHY-decodable
    (per-frame SNR available); the Jaguar3 PMAC carrier is a spectral stimulus,
    not a frame stream — hence the probe's beacon feed.

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