Lab042: transmit an image over the air, add the air loopback mode

First transmission through a real radio path in the project. A 4756-byte
JPEG went from transmitter to receiver over two antennas 60 cm apart at
225 MHz and reassembled byte-identical, reproduced across three runs.

The cable loopback still needs an SMA male-to-male adapter that has not
arrived, so this air run is a track verification, not the controlled
measurement: room levels are not held the way a cable holds them. Its
purpose was to exercise the device wrapper the models cannot cover.

Four defects the software model passed but the hardware exposed, each
reproduced and fixed:

- a 566 528-sample frame did not fit the 262 144 receive buffer, and the
  sample stream is not continuous across separate reads, so a frame on the
  seam was destroyed. Reception now uses one buffer sized to the frame
- carrier correction was always applied, but with a shared reference the
  true offset is near zero and the 80-symbol marker estimate is noise;
  correcting by it rotated phase along the whole frame, 65% bit errors
  measured. Ported should_apply_cfo_correction from Lab023
- receiver gain of 20 dB left the signal at 0.0003 of full scale. Levels
  set by measurement: RX 50 dB, TX -10 dB give zero bit errors with margin
- the error-vector metric returned zero on a failed receive, reporting a
  perfect signal where there was none, the same class of defect as the
  written-in figures of Lab041. Replaced with an explicit not-computed value

The air TX gain ceiling is now derived from the link budget by antenna
distance rather than a constant. Three loopback modes: software, cable,
air. PROJECT_LOG entry 019 records the run and the defects.

Software and cable results stay reproducible; 125 tests pass, gate passes.

data/processed/lab042/lab042_summary.csv

@@ -0,0 +1,2 @@
+loopback_mode,path_description,antenna_distance_cm,source_image,image_width,image_height,jpeg_quality,jpeg_bytes,fragment_data_size,fragment_count,packet_bytes_max,packet_bytes_min,frame_bits_max,frame_bits_min,symbol_rate,samples_per_symbol,sample_rate_hz,carrier_hz,transmitted_samples,air_time_seconds,frames_received,frames_failed_crc,fragments_recovered,fragment_delivery_fraction,image_reassembled,image_identical,received_image_bytes,transfer_seconds,channel_bitrate_kbps,wall_clock_bitrate_kbps,mean_correlation_score,mean_carrier_offset_hz,mean_error_vector_magnitude,tx_gain_db,rx_gain_db,attenuator_db
+air,"две антенны по воздуху, расстояние 60 см, аттенюатора нет",60.0,data/raw/lab011_scene.jpg,320,180,30,4756,512,10,536,172,4384,1472,20000,128,2560000,225000000,5292544,2.0674,10,0,10,1.0,True,True,4756,16.637855200009653,18.40379220276676,2.286833221146072,0.9995110413728169,-0.3681907269061185,0.00898132918175413,-10.0,50.0,0.0