PR-AB.b expanded commit 6: subframe_pulse strip + stale MCU DIAG cleanup

Final cleanup pass for the PR-AB.b expanded bundle.

chirp_scheduler.v / radar_receiver_final.v: delete the unused subframe_pulse
output + its sibling subframe_id port. Both were declared on the scheduler
and bound at the radar_receiver_final.sched instance, but no downstream
module read them -- doppler_processor counts sub-frame boundaries
internally from new_chirp_frame + CHIRPS_PER_SUBFRAME=16. The stale
"// doppler picks up in PR-F" comment was aspirational; PR-F never wired
it. subframe_id demoted from output port to internal reg (still consumed
by the FSM's next_enabled_subframe helper).

tb_chirp_scheduler_handshake.v: drop the matching observer wires + port
bindings.

main.cpp: replace the F-2.1 "host_radar_mode = 2'b01 (auto-scan, FPGA-owned
chirp dispatch)" boot DIAG + 8-line comment block with a short comment
noting the mode register was retired in commit 1. The DIAG was asserting a
register that no longer exists.

Regression:
- FPGA iverilog: 43/0/0 (tb_chirp_scheduler_handshake 16/16)
- MCU: 51/0 (GPS) + 34/0 (AGC/safety/gap-3)

Closes the PR-AB.b expanded bundle (commits 1-6) on feat/dual-range-v2.

9_Firmware/9_2_FPGA/tb/tb_chirp_scheduler_handshake.v

@@ -40,10 +40,8 @@ reg         handshake_enable = 1'b0;
 
 wire [1:0]  wave_sel;
 wire        chirp_pulse;
-wire        subframe_pulse;
 wire        frame_pulse;
 wire [5:0]  chirp_counter;
-wire [1:0]  subframe_id;
 wire [15:0] cfg_chirp_cycles;
 wire [15:0] cfg_listen_cycles;
 wire [15:0] cfg_guard_cycles;
@@ -66,10 +64,8 @@ chirp_scheduler dut (
     .host_handshake_enable        (handshake_enable),
     .wave_sel                     (wave_sel),
     .chirp_pulse                  (chirp_pulse),
-    .subframe_pulse               (subframe_pulse),
     .frame_pulse                  (frame_pulse),
     .chirp_counter                (chirp_counter),
-    .subframe_id                  (subframe_id),
     .cfg_chirp_cycles             (cfg_chirp_cycles),
     .cfg_listen_cycles            (cfg_listen_cycles),
     .cfg_guard_cycles             (cfg_guard_cycles),