Implement the transmit/generate side as streaming GNU Radio blocks,
complementing the existing receive chain. Each block maps to a physical
instrument on CuriousMarc's Keysight bench:
pcm_frame_source - PCM bit stream generator (sync_block + FrameSourceEngine)
nrz_encoder - bits to NRZ waveform (+1/-1) with upsampling
bpsk_subcarrier_mod - NRZ x cos(1.024 MHz) BPSK modulator
fm_voice_subcarrier_mod - 1.25 MHz FM test tone source
pm_mod - phase modulator: exp(j * deviation * input)
usb_signal_source - convenience wrapper wiring all blocks together
Includes GRC YAML definitions for all blocks under [Apollo USB] category,
49 new tests (271 total, all passing), and a loopback test that validates
the full TX->RX round trip including frame recovery with 30 dB AWGN.
