ft2232h: add frame drop counter (AUDIT-C12) + cfar RMW cadence guard (AUDIT-S22)

AUDIT-C12: usb_data_interface_ft2232h had a misleading single-buffer comment
that overstated the timing slack and referenced a frame_ack_toggle CDC that
was never implemented. Re-verified actual numbers: at 178 fps the slack is
1.14 ms (20%), not "much shorter than gap". No data corruption today (write
order matches read order, addresses don't collide), but frame_complete
firing while WR_FSM is still draining the previous frame causes silent
frame drops via the missed frame_ready_toggle edge.

Fix is instrumentation, not architectural rework: add wr_done_toggle
(ft_clk -> clk CDC) on WR_DONE -> WR_IDLE, track frame_pending in clk
domain, count drops in 7-bit saturating frame_drop_count, surface in
unused upper 7 bits of status_words[5]. Host now has visibility into the
failure mode if margin ever shrinks (faster frame rate or USB bandwidth
shortfall). Replaced misleading comment with corrected timing breakdown.

AUDIT-S22: cfar_ca emits one detection per 3 cycles (THR/MUL/CMP); the
detection RMW takes 3 cycles. Match by construction today, fragile against
any CFAR speedup. Added a header comment in cfar_ca.v documenting the
dependency, and a SIMULATION-only assertion in usb_data_interface_ft2232h.v
that fires [ASSERT FAIL] AUDIT-S22 if cfar_valid arrives while RMW busy.
Catches silent-drop regressions in the test suite.

Verification: new tb_ft2232h_frame_drop.v with 5 scenarios (no drops /
stalled drops / multi-drop / recovery / saturation at 127) - 10/10 PASS.
Quick regression 31/31 PASS (was 30/30; +1 new test, 0 regressions).

9_Firmware/9_2_FPGA/run_regression.sh

@@ -550,6 +550,10 @@ run_test "FFT AXI Bridge tready handshake (AUDIT-C10)" \
     tb/tb_fft_engine_axi_bridge.vvp \
     tb/tb_fft_engine_axi_bridge.v fft_engine_axi_bridge.v
 
+run_test "FT2232H Frame Drop Counter (AUDIT-C12)" \
+    tb/tb_ft2232h_frame_drop.vvp \
+    tb/tb_ft2232h_frame_drop.v usb_data_interface_ft2232h.v
+
 echo ""
 
 # ===========================================================================