fix: resolve 3 cross-layer bugs (status_words truncation, mode readback, buffer overread)

Bug 1 (FPGA): status_words[0] was 37 bits (8+3+2+5+3+16), silently
truncated to 32. Restructured to {0xFF, mode[1:0], stream[2:0],
3'b000, threshold[15:0]} = 32 bits exactly. Fixed in both
usb_data_interface_ft2232h.v and usb_data_interface.v.

Bug 2 (Python): radar_mode extracted at bit 21 but was actually at
bit 24 after truncation — always returned 0. Updated shift/mask in
parse_status_packet() to match new layout (mode>>22, stream>>19).

Bug 3 (STM32): parseFromUSB() minimum size check was 74 bytes but
9 doubles + uint32 + markers = 82 bytes. Buffer overread on last
fields when 74-81 bytes passed.

All 166 tests pass (29 cross-layer, 92 GUI, 20 MCU, 25 FPGA).

9_Firmware/9_3_GUI/test_radar_dashboard.py

@@ -130,8 +130,8 @@ class TestRadarProtocol(unittest.TestCase):
         pkt = bytearray()
         pkt.append(STATUS_HEADER_BYTE)
 
-        # Word 0: {0xFF, 3'b0, mode[1:0], 5'b0, stream[2:0], threshold[15:0]}
-        w0 = (0xFF << 24) | ((mode & 0x03) << 21) | ((stream & 0x07) << 16) | (threshold & 0xFFFF)
+        # Word 0: {0xFF[31:24], mode[23:22], stream[21:19], 3'b000[18:16], threshold[15:0]}
+        w0 = (0xFF << 24) | ((mode & 0x03) << 22) | ((stream & 0x07) << 19) | (threshold & 0xFFFF)
         pkt += struct.pack(">I", w0)
 
         # Word 1: {long_chirp, long_listen}