Ch443e: binomial 4th line buffer + prefetch lead-2 (fix lookahead underflow)

The Ch443d board per-frame diagnostic identified the real displayed-frame
failure as a BINOMIAL vertical-lookahead starvation: displaying source
row r while interpolating r-1/r/r+1, the prefetch led by only one row
(next_fetch <= disp_row+1), so the lookahead row r+1 was still in flight
when the 3x3 filter read it (board: scan_y=33, nf_v=34, cause_lookahead=1,
read-error=0, deterministic every frame).

Fix (BINOMIAL_3X3_FILTER only; legacy 2/3-buffer, lead-1 paths unchanged):
- Add a 4th rotating line buffer (lb3) with its own RAM-local write/read/
  cache registers. The 3x3 filter needs r-1/r/r+1 resident (3 buffers), so
  leading by 2 (fetch r+2 while displaying r) without overwriting r-1
  requires a 4th buffer.
- Prefetch lead-2 for binomial: disp_row_limit_e = disp_row+2. The in-flight
  r+2 lands in the 4th buffer (b+2 mod 4), always distinct from prev/cur/next
  (b-1/b/b+1 mod 4), so it never clobbers a row being read.
- Modulo-4 rotation everywhere: V_SOURCE_BUF%4, stretch_buf_q, next_fetch_buf,
  reset alignment at V_SOURCE_START, and the read-cache prev/cur/next case
  extended to 4 branches with (b-1)/b/(b+1) mod 4 selection + first/last-row
  clamps preserved.

New tb_gs_scanout_binomial_lookahead reproduces the board condition under
realistic EMIF latency (LAT=7) + backpressure and proves: NO binomial
lookahead underflow, correct 3x3 output across modulo-4 wrap + clamps (full
oracle, 1280 px), and coverage that mid-frame rows past V_SOURCE_START+1
with vphase!=0 were exercised under prefetch pressure.

All green: binomial (4-buffer, identical output), lookahead (new),
scanout_lb {,_hstretch,_psm32_256,_fb}, scanout_restart, scanout_diag,
ps2_hps_bridge, and the complete f52 replay BYTE-IDENTICAL (Z 0/307200,
COLOR 0/245760). Also commits the Ch443d board evidence that identified
this defect. No Quartus/board/push from here.

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

sim/Makefile

@@ -1281,6 +1281,15 @@ 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_binomial_lookahead: dirs
+	@echo "=== build tb_gs_scanout_binomial_lookahead ==="
+	$(IVERILOG) $(IVERILOG_FLGS) \
+	    -o $(BUILD_DIR)/tb_gs_scanout_binomial_lookahead.vvp \
+	    -s tb_gs_scanout_binomial_lookahead \
+	    $(RTL_SRCS) $(TB_ROOT)/gif_gs/tb_gs_scanout_binomial_lookahead.sv
+	@echo "=== run tb_gs_scanout_binomial_lookahead ==="
+	@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_scanout_binomial_lookahead.vvp
+
 tb_gs_scanout_diag: dirs
 	@echo "=== build tb_gs_scanout_diag ==="
 	$(IVERILOG) $(IVERILOG_FLGS) \