radiotap TX: single shared HT MCS decode across all three generations

[josephnef]

Summary

Follow-ups from the #291 review, in one pass:

• Jaguar1 adopts decode_radiotap_mcs_field() — the last private copy of the HT MCS known/flag reading is gone, so the three generations' send_packet parsers genuinely can't diverge. Behavior-identical: bwidth starts at 20 MHz, so J1's explicit BW_20/20L/20U branch was a no-op, and its ldpc/stbc/sgi semantics already matched the shared decode.
• Named constants — the decoder spells its bit tests with IEEE80211_RADIOTAP_MCS_HAVE_FEC/FEC_LDPC/HAVE_STBC/STBC_MASK/STBC_SHIFT/SGI instead of raw masks and shifts.
• Duplicate include dropped — RadiotapTxFlags.h was included twice in RtlJaguar3Device.cpp.
• Selftest: new raw 3-byte MCS field case (BW code 20U, STBC stream count 2, LDPC, no SGI) pinning the decoder's bit positions independently of what build_stream_radiotap can emit — a direct guard on the constants refactor.

Testing

• ctest 21/21 green.
• Bench-verified on air (streamtx MCS1/LDPC/STBC → rx.txhit decode):
  • 8812AU (J1 — the swapped path) → 8822B RX, ch6: 107/107 frames ldpc=1 stbc=1, rate MCS1.
  • 8812EU (J3 regression on the refactored decoder) → 8812AU RX, ch36: 19/19 ldpc=1 stbc=1.

🤖 Generated with Claude Code