Ch443f: coherent Gray readiness CDC + qualified scanout underflow detector

Harden the LPDDR scanout underflow diagnostic (Codex direction). Keeps the
Ch443e 4-buffer + lead-2 fix; adds no 5th buffer.

RTL (gs_lpddr_scanout_lb):
- Replace the raw-binary next_fetch readiness sync with a reset-aware GRAY
  code. next_fetch is monotonic between frames, so one Gray bit changes per
  increment; the 2-FF-synced + decoded nf_v is always a real prior frontier
  (monotone, burst-safe), never a torn multi-bit combination. (A plain
  toggle-per-change handshake dropped bursts when two increments landed in one
  sync window; that is why the earlier attempt under-read nf_v and false-tripped.)
  fs_edge_v overrides the lone multi-bit reset transient.
- Qualify the underflow: only a miss persisting >= QUAL_CYCLES (4) sets sticky
  underflow. uf_pmax_q records the longest streak and uf_qual_q whether any
  qualified, so a host can distinguish a 1-cycle CDC lag from a real late row.
- Atomic snapshot: scan_y/nf_v/pmax/causes/vphase/line_valid all latched the
  same video cycle on the first qualified miss.

ABI: 0x120 adds [15:12]=live pmax (2-FF synced via scan_diag_pmax_i). 0x124
[29:20] now carries pmax-at-capture (was nf_s0). Bridge dst reg kept 10-bit.

SDC: scanout diag bundle source count 37 -> 31 (nf_s0[10] -> pmax[4]); new
async-in cut + max_skew/net_delay for the next_fetch Gray CDC; stage-0 cut for
the live pmax sync.

Tests: new tb_gs_scanout_cdc_qual (async-clock focused: no false event from the
readiness transition or ordinary sync latency; sub-QUAL transient does not
qualify; genuine late row qualifies with a self-consistent atomic snapshot;
frame reset + mod-4 reuse re-arm the detector). tb_gs_scanout_diag updated for
the pmax field. Tie off scan_diag_pmax_i (+ pre-existing clut_* gap from the
fog baseline) in the four .* bridge/integration TBs.

Regressions green: scanout (cdc_qual, binomial_lookahead, diag, restart,
lpddr_scanout_lb x3), regbuf (r/aw/w), bridge + 3 pad integration TBs, and the
f52 top-level golden FB (Z 0/307200, COLOR 0/245760 mismatch, drops=0).

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

sim/tb/integration/tb_ee_pad_buffer_branch.sv

@@ -256,6 +256,11 @@ module tb_ee_pad_buffer_branch;
     logic        scan_diag_uf_i = 1'b0, scan_diag_rderr_nz_i = 1'b0, scan_diag_valid_i = 1'b0;
     logic [29:0] scan_diag_first_i = 30'd0;
     logic [6:0]  scan_diag_stat_i = 7'd0;
+    logic [3:0]  scan_diag_pmax_i = 4'd0;
+    // Pre-existing clut_stage/commit bridge outputs (fog-baseline ba74bbd); nets so .* binds.
+    wire [7:0]  clut_stage_waddr_o; wire [31:0] clut_stage_wdata_o; wire clut_stage_we_o;
+    wire        clut_commit_tgl_o;  wire [31:0] clut_expected_crc_o;
+    logic       clut_busy_i = 1'b0, clut_done_tgl_i = 1'b0; logic [31:0] clut_crc_i = 32'd0;
     ps2_hps_bridge u_bridge (
         .clk            (bclk),
         .reset_n        (breset_n),