tx: programmatic runtime TX-mode API; drop DEVOURER_TX_HT_MCS gate

[josephnef]

Problem

RtlJaguarDevice::send_packet only wired a radiotap HT-MCS index into the on-air rate when DEVOURER_TX_HT_MCS was set in the environment. Without it, a correctly-formed HT radiotap silently fell back to the MGN_1M default (1 Mbps CCK, clamped to 6 Mbps legacy at 5 GHz). Two things wrong: a library deciding TX behaviour from a CLI env var, and a valid radiotap being ignored by default. The demo compounded it by hand-patching radiotap bytes through a swarm of per-knob env vars.

Change

• Radiotap is authoritative per-packet — the HT-MCS index is honoured unconditionally (the DEVOURER_TX_HT_MCS gate is removed).
• New runtime API — devourer::TxMode (src/TxMode.h) + RtlJaguarDevice::SetTxMode() / ClearTxMode(). The mode is a configurable default, applied only when a frame's radiotap carries no rate (replacing the hardcoded MGN_1M); per-packet radiotap always wins. Mirrors the existing SetTxPowerOverride pattern (pure state, no USB I/O).
• Consolidated CLI — the per-knob DEVOURER_TX_MCS/_VHT/_VHT_MCS/_VHT_NSS/_LDPC/_STBC/_BW env vars collapse into one string:

  DEVOURER_TX_RATE=<rate>[/<bw>][/SGI][/LDPC][/STBC]
    <rate> = 6M..54M | MCS0..MCS31 | VHT1SS_MCS0..VHT4SS_MCS9
    <bw>   = 20|40|80|160        (default 20)
  

  parsed by parse_tx_mode_env() into a TxMode. e.g. MCS7/40/SGI, VHT2SS_MCS3/80/LDPC, 6M.
• StreamRateCfg → devourer::TxMode; WiFiDriverTxDemo sends a rate-less beacon and drives the mode via SetTxMode. Stream demos migrated.
• tests/regress.py, tests/bench_onair.py, tests/thermal_gain_sweep.py and the docs migrated to DEVOURER_TX_RATE; corrected the precoder note that claimed send_packet never reads the HT MCS index.

DEVOURER_TX_PAYLOAD_BYTES and DEVOURER_TX_PWR_* are unchanged (size / power, not mode).

Verification

• cmake --build build -j clean across all targets; ctest passes (stream_stdin_binary).
• On air (8812 TX → 8814 monitor, ch6, no DEVOURER_TX_HT_MCS):
  • DEVOURER_TX_RATE=MCS7 → witness decodes 11n … 65.0 Mb/s MCS 7 20 MHz.
  • unset → witness decodes 6.0 Mb/s … 11g (the TxMode default, not 1M CCK).

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