fix(fpga): PR-Q.1 — stagger MEDIUM PRI 175→161 µs for 3-PRI Doppler CRT (C-5)

Bumps RP_DEF_MEDIUM_LISTEN_CYCLES 17000 → 15600 so MEDIUM PRI = 161 µs,
distinct from SHORT (175 µs) and LONG (167 µs). Three coprime PRIs let
the host run 3-PRI Chinese-Remainder unfolding on Doppler aliases beyond
the per-sub-frame ±~41 m/s unambiguous range — closes the FPGA half of
audit C-5 (PR-F Doppler ambiguity unfolding).

Stagger choice (proposal B):
  SHORT  175 µs — chirp 1   + listen 174
  MEDIUM 161 µs — chirp 5   + listen 156   (PR-Q, was 175)
  LONG   167 µs — chirp 30  + listen 137

In 3 km mode LONG is blind (4500 m blind zone) → SHORT-vs-MEDIUM
(Δ=14 µs / 8 %) is the operative pair; in 20 km mode MEDIUM-vs-LONG
(Δ=6 µs / 4 %) carries the long-range slice that has SNR for both.
Listens picked to differ by ≥5 % so the alias resolver is robust against
the 5.1 m/s/bin Doppler quantization.

Architecture is unchanged — chirp_scheduler.v already takes per-waveform
host_*_listen_cycles. doppler_processor.v / cfar_ca.v are PRI-agnostic
and just tag Doppler outputs with sub_frame ID; host-side CRT lives in
v7/processing.py (PR-Q.5, follow-on).

Files:
  radar_params.vh:240        RP_DEF_MEDIUM_LISTEN_CYCLES 17000 → 15600
  radar_params.vh:217-228    block comment: stagger rationale + Δ math
  radar_system_top.v:273     port-list comment: default 17000 → 15600
  radar_system_top.v:278-282 staggered-PRI block comment: 3-ladder PRI
  doppler_processor.v:25-30  reference v7/processing.py CRT unfolder
  tb/tb_radar_receiver_final.v:199-202  list MEDIUM=15600 in real-values

Validation: full iverilog regression 42 PASS / 0 FAIL / 1 SKIP (pre-
existing scipy availability) — same baseline as post-PR-O.8. No TB
default-value asserts touched (tb_system_opcodes / tb_usb_protocol_v2
both use literal 16500 for opcode 0x18 round-trip).

Follow-on: PR-Q.2 (cosim T_PRI_MEDIUM align + golden regen), PR-Q.4-7
(v7 GUI 3-PRI CRT unfolder + AMBIGUOUS confidence display), PR-Q.8
(memory close-out). MCU executeChirpSequence is live but PRI-agnostic
in production mode 2'b01 (FPGA auto-scan) — pre-existing 2-ladder
staleness vs chirp-v2 3-ladder, defer to PR-H or dedicated MCU PR.

9_Firmware/9_2_FPGA/radar_params.vh

@@ -214,8 +214,15 @@
 // Reset defaults for host-configurable timing registers.
 // Match radar_mode_controller.v parameters and main.cpp STM32 defaults.
 //
-// 3-LADDER (3 km build): SHORT 1 µs, MEDIUM 5 µs, LONG 30 µs. Same listen
-// budget across waveforms (~175 µs PRI) keeps Doppler resolution uniform.
+// 3-LADDER (3 km build): SHORT 1 µs, MEDIUM 5 µs, LONG 30 µs.
+// PRI ladder is intentionally STAGGERED across waveforms — SHORT 175 µs,
+// MEDIUM 161 µs, LONG 167 µs (PR-Q). Three coprime PRIs let the host run
+// 3-PRI Chinese-Remainder unfolding on Doppler aliases (see C-5 in the
+// 2026-04-29 audit). In 3 km mode LONG is blind (4500 m blind zone) so
+// SHORT-vs-MEDIUM (Δ=14 µs / 8 %) is the operative pair; in 20 km mode
+// MEDIUM-vs-LONG (Δ=6 µs / 4 %) carries the long-range slice that has
+// SNR for both. Picking listen cycles to differ by ≥5 % keeps the alias
+// resolver robust against the 5.1 m/s/bin Doppler quantization.
 // LONG kept on 50T as legal-but-unused so 200T spin-up doesn't need a
 // second wave through the codebase.
 
@@ -228,9 +235,9 @@
 
 // 3-ladder defaults — added in PR-A, consumed by chirp_scheduler in PR-D.
 `define RP_DEF_SHORT_CHIRP_CYCLES_V2   100     // 1 µs at 100 MHz (was 0.5 µs)
-`define RP_DEF_SHORT_LISTEN_CYCLES_V2  17400   // PRI 175 µs - chirp - guard slack
+`define RP_DEF_SHORT_LISTEN_CYCLES_V2  17400   // SHORT PRI 175 µs (chirp 1 + listen 174)
 `define RP_DEF_MEDIUM_CHIRP_CYCLES     500     // 5 µs at 100 MHz
-`define RP_DEF_MEDIUM_LISTEN_CYCLES    17000   // PRI 175 µs - chirp - guard slack
+`define RP_DEF_MEDIUM_LISTEN_CYCLES    15600   // MEDIUM PRI 161 µs (chirp 5 + listen 156, PR-Q stagger)
 // LONG defaults reuse RP_DEF_LONG_CHIRP_CYCLES / RP_DEF_LONG_LISTEN_CYCLES
 `define RP_DEF_CHIRPS_PER_SUBFRAME     16      // 16 per sub-frame, 3 sub-frames = 48 frame
 `define RP_DEF_SUBFRAME_ENABLE         3'b111  // SHORT|MEDIUM|LONG all on by default