Follow-ups left open by the Jaguar3 wide-TX IGI floor (#484) #485

Open items from #484 (Jaguar3 40/80 MHz inject: wide_tx_igi_floor on both dies, 8822C DISDATAFB on wide frames), none of them blocking the merge:

• The floor's RX-sensitivity cost is an estimate. The default 0x35 is 21 IGI steps over the DFS floor and 19-23 over the non-DFS window, quoted as "~1 dB per step" and never measured. This is the number an FPV uplink actually pays on the wide-transmitting side. Measure it: same transmitter at a fixed attenuation into a wide-TX session with the floor at 0x35 vs DEVOURER_WIDE_TX_IGI_FLOOR=0 (RX-only arming never happens, so the control is a session that sends one wide frame then listens), delivery vs attenuation, both dies.
• No margin story behind the default. 0x35 is the lowest value that cleared the four units measured, not a margin over a characterized edge: one 8812CU clears at 0x24, the second hangs at 0x26 and airs ~1.5 k frames then wedges at 0x2c, and was never bisected between 0x2c and 0x35. Bisect that unit (0x2e/0x30/0x32, silent 144(-), MCS3/40, the 257 submitted / 8 failed / 0 aired shape as the fail criterion) so the default is "edge + N" rather than "the highest value anyone needed".
• The real lever is unidentified. The hold is the receiver parking, on a quiet channel, in a gain state where the secondary 20 MHz never reads idle; any received traffic re-runs the AGC and clears it, DEVOURER_DIS_CCA (MAC CCA + EDCCA + sec20/40/80 gates) is inert against it, and the vendor phydm has no bandwidth-raised DIG bound (phydm_dig_abs_boundary_decision pins DFS at DIG_MIN_DFS at every width). The IGI floor is a workaround that buys TX at the cost of RX gain. The idle-vs-live register diff in docs/experiments/8822c-ht40-tx-stall.md (RF 0x08/0x58, CCK/AGC status 0x1a30-0x1a68) is where to look for a state the secondary CCA path reads; a lever that re-arms the AGC without raising IGI would make the floor unnecessary.
• 80 MHz only partially validated. CU -> EU on 149(+) at VHT1SS_MCS3/80 with the old 0x26 floor ran 470 / 10 / 209; the 0x35 default has not been flown at 80 MHz on either bench. Re-run on a silent 80 MHz block with the default and with the floor off.
• 2.4 GHz HT40 (ch 6). The CU drained on the TX side but the witness received none of it, before and after the change. Unexplained; not a 5 GHz-only feature claim until it is.
• The 8812EU module's scattered send failures. On the second bench the BL-M8812EU2 drops single sends beside coex: tick failed (rtw_read: iostream error) in otherwise clean runs (13/8501 on 144(-), 64/7316 on busy 36(+)); the CU on the same hub never does. Not the stall shape, and probably that unit's USB wiring (bare module on a bench supply), but it has not been proven so: swap port / cable once and see whether the errors follow the module.

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