Ch442: A+B scanout diagnostic (split LPDDR_STATUS[5] + first-failure snapshot)

Read-only diagnostic to disambiguate the three causes folded into
LPDDR_STATUS[5]. bit5 (0x02C) semantics are UNCHANGED.

- gs_lpddr_scanout_lb: register a live (rd_errs!=0) flag in the emif
  domain (the raw counter never crosses), and capture the FIRST raw
  underflow of each video-source-enabled session as a bundled-data
  snapshot (scan_y/nf_v/nf_s0 + base-vs-lookahead cause + line_valid +
  vphase), held stable until !enable. The existing sticky underflow_v
  latch, fetch FSM, pixel path and arbitration are untouched.
- ps2_hps_bridge: independently 2-FF sync the two split live flags;
  latch the snapshot on the rising synced-valid edge (coherent bundled
  data). New read-only regs 0x120 SCAN_DIAG_STATUS / 0x124
  SCAN_DIAG_FIRST (window addr[37:5]==9); 0x118/0x11C reserved slots
  untouched.
- top: drive 5 diag nets per scanout arm like scan_err_w; bit5 assign
  unchanged.
- tb_gs_scanout_diag (new): provokes AXI RRESP error, cold-start row-0
  starvation, first-failure capture + coherent bridge readback, and
  clear-via-video-source-disable (17/17 checks).
- Tie off the new bridge inputs in the four .*-instantiating TBs.

Sim set all PASS: focused TB, tb_ps2_hps_bridge, scanout_lb
{binomial,hstretch,psm32_256}, and the complete f52 replay (FB
byte-identical: Z 0/307200, COLOR 0/245760; sum32=0xaad0b94d).

No fix / gray-code / persistence-filter / scanout checksum. No Quartus,
no board, no push.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

sim/Makefile

@@ -1261,6 +1261,15 @@ tb_gs_lpddr_scanout_lb_binomial: dirs
 	@echo "=== run tb_gs_lpddr_scanout_lb_binomial ==="
 	@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_lpddr_scanout_lb_binomial.vvp
 
+tb_gs_scanout_diag: dirs
+	@echo "=== build tb_gs_scanout_diag ==="
+	$(IVERILOG) $(IVERILOG_FLGS) \
+	    -o $(BUILD_DIR)/tb_gs_scanout_diag.vvp \
+	    -s tb_gs_scanout_diag \
+	    $(RTL_SRCS) $(TB_ROOT)/gif_gs/tb_gs_scanout_diag.sv
+	@echo "=== run tb_gs_scanout_diag ==="
+	@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_scanout_diag.vvp
+
 tb_gs_texture_cache: dirs
 	@echo "=== build tb_gs_texture_cache ==="
 	$(IVERILOG) $(IVERILOG_FLGS) \