Ch443c: per-frame scanout diagnostic + frame-restart latency fix

The Ch443 board A+B diagnostic captured a WARM-UP miss, not a displayed-
frame miss: the line-buffer reader is enabled by video_src_emif
immediately, but the HDMI mux only switches to it at the next vsync, and
the diagnostic (cleared only on !enable) froze that pre-display capture.
scan_y=32/nf=32 was the expected pre-display warm-up, and line_valid is
sticky so it only meant SOME row had loaded, not that row 32 was valid.

Two fixes (all accepted Ch443 timing repairs kept: AW buffer, F_SETTLE +
drain multicycle, tile max-skew, monolithic tex_mem):

1. Per-frame diagnostic: clear diag_valid_q on fs_edge_v as well as
   !enable (mirrors underflow_v). Discards the warm-up capture and
   records the first miss, if any, AFTER the real frame boundary.

2. Frame-restart latency: in L_R, after the single-beat response is
   accepted, if fs_pending || fs_edge_e, abandon the remainder of the
   obsolete row -- no beat commit, no next old-row AR, no publish/
   increment -- and return to L_IDLE, which restarts at V_SOURCE_START.
   Protocol-safe (the accepted AXI transaction is complete); removes up
   to a full row of restart latency during vertical blanking, so the
   restarted prefetch leads the first displayed row.

New tb_gs_scanout_restart proves: mid-fetch frame-start accepts the
in-flight response, issues NO further old-row AR, restarts at row 32,
loads rows 32/33 before active consumption, and no post-restart
underflow. Regressions green: scanout_lb {,_binomial,_hstretch,
_psm32_256,_fb}, scanout_diag (per-frame), ps2_hps_bridge, and the
complete f52 replay BYTE-IDENTICAL (Z 0/307200, COLOR 0/245760).

Also commits the previously-untracked Ch443 board A+B evidence
(docs/hardware/ch443_board_validation/: verdict, 3-session raw, board
FB, RBF sha). No Quartus/board/push from here.

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

sim/Makefile

@@ -1280,6 +1280,15 @@ tb_gs_scanout_diag: dirs
 	@echo "=== run tb_gs_scanout_diag ==="
 	@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_scanout_diag.vvp
 
+tb_gs_scanout_restart: dirs
+	@echo "=== build tb_gs_scanout_restart ==="
+	$(IVERILOG) $(IVERILOG_FLGS) \
+	    -o $(BUILD_DIR)/tb_gs_scanout_restart.vvp \
+	    -s tb_gs_scanout_restart \
+	    $(RTL_SRCS) $(TB_ROOT)/gif_gs/tb_gs_scanout_restart.sv
+	@echo "=== run tb_gs_scanout_restart ==="
+	@cd $(TRACE_DIR) && $(VVP) $(BUILD_DIR)/tb_gs_scanout_restart.vvp
+
 tb_gs_texture_cache: dirs
 	@echo "=== build tb_gs_texture_cache ==="
 	$(IVERILOG) $(IVERILOG_FLGS) \