Ch442 review fixes: fs_edge parity, dest snap_valid, sync attrs + SDC

Codex review containment/observability fixes (no behavior change, no redesign):

- scanout_lb: add !fs_edge_v to the diagnostic predicate so capture
  matches the underflow latch's frame-start CLEAR priority exactly
  (no capture on an fs_edge cycle the real latch suppresses).
- bridge: expose a destination snap_valid_q as 0x120[0] — set ON the
  payload-capture edge, cleared on synced source-valid deassert — so
  valid never leads the bundle by a cycle (was the middle sync stage).
- bridge: forced-synchronizer (SYNCHRONIZER_IDENTIFICATION FORCED) +
  dont_merge/preserve on the underflow/read-error/valid chains;
  preserve on the bundle capture regs (both domains).
- SDC: stage-0 async cuts on the three sync[0] inputs + the 37-bit
  stable bundle hold-false-path + 2ns max_skew + 2ns net_delay, with
  fail-closed src==37 / dst!=0 count checks (tile_ram_cdc idiom).
- tb_gs_scanout_diag: +fs_edge-suppression monitor (with coverage that
  the coincidence is exercised), +valid-ordering monitor, +snapshot
  stability after later misses, +production DUT (V_SOURCE_START=32,
  stretch, linear) proving cold-start scan_y=32, +cause/phase packing.
  32/32 checks pass.
- doc: production cold start is source row 32 (not 0), base+lookahead
  may both assert, and one snapshot narrows but does not prove
  starvation vs next_fetch CDC-lag.

Sim set all PASS: focused TB, tb_ps2_hps_bridge, scanout_lb
{binomial,hstretch,psm32_256}, complete f52 replay (FB byte-identical
Z 0/307200, COLOR 0/245760). No Quartus, board, push, or scanout
behavior change.

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

synth/de25_nano/top_psmct32_raster_demo/de25_nano_psmct32_raster_demo_top.sdc

@@ -353,6 +353,44 @@ set osd_dst_n [get_collection_size $osd_cfg_dst]
 if { $osd_src_n == 0 } { post_message -type warning "Ch357 SDC WARN: osd_cfg src pattern matched 0 keepers (renamed? osd_cfg false-path NOT applied)" }
 if { $osd_dst_n == 0 } { post_message -type warning "Ch357 SDC WARN: osd_cfg dst pattern matched 0 keepers (renamed? osd_cfg false-path NOT applied)" }
 set_false_path -from $osd_cfg_src -to $osd_cfg_dst
+
+# ============================================================================
+# Ch442 (Codex) — scanout A+B diagnostic CDC. gs_lpddr_scanout_lb (u_lpddr_scan_lb)
+# drives the diagnostic into ps2_hps_bridge (u_hps_bridge) across async domains:
+#   (a) THREE single-bit synchronizer chains — underflow (video_clk), read-error
+#       (emif_clk), valid (video_clk) — 2/3-FF (* FORCED, dont_merge, preserve *).
+#       Cut the async INTO stage 0 only (setup+hold); the sync[0]->sync[1..] MTBF
+#       leg stays timed. Nonzero-check WARN (the whole path is absent in non-LPDDR
+#       profiles), not fatal.
+#   (b) the 37-bit STABLE first-failure bundle {scan_y[10], nf_v[10], nf_s0[10],
+#       vphase[4], base, lookahead, line_valid}. It is written once per enabled
+#       session and held quiescent until the bridge captures it on the synced-valid
+#       edge -> the raw HOLD check is false; cut HOLD only + bound bit spread
+#       (set_max_skew 2ns) + bound net arrival (set_net_delay -max 2ns), same recipe
+#       as the tile_ram_cdc bundle above. FATAL count checks so a rename can't
+#       silently orphan the exception (src==37 exact; dst nonzero).
+foreach s {scan_diag_uf_sync scan_diag_rderr_sync scan_diag_valid_sync} {
+    set s0 [get_keepers -nowarn "*u_hps_bridge|$s\[0\]"]
+    if { [get_collection_size $s0] == 0 } {
+        post_message -type warning "Ch442 SDC WARN: $s\[0\] matched 0 keepers (renamed / no LPDDR scanout in profile) — stage-0 cut NOT applied"
+    } else {
+        set_false_path -to $s0
+    }
+}
+set scan_diag_src [get_keepers -nowarn {*u_lpddr_scan_lb|diag_scan_y_q[*] *u_lpddr_scan_lb|diag_nf_v_q[*] *u_lpddr_scan_lb|diag_nf_s0_q[*] *u_lpddr_scan_lb|diag_vphase_q[*] *u_lpddr_scan_lb|diag_base_q *u_lpddr_scan_lb|diag_look_q *u_lpddr_scan_lb|diag_lv_q}]
+set scan_diag_dst [get_keepers -nowarn {*u_hps_bridge|scan_diag_scan_y_q[*] *u_hps_bridge|scan_diag_nf_v_q[*] *u_hps_bridge|scan_diag_nf_s0_q[*] *u_hps_bridge|scan_diag_vphase_q[*] *u_hps_bridge|scan_diag_base_q *u_hps_bridge|scan_diag_look_q *u_hps_bridge|scan_diag_lv_q}]
+set scan_diag_src_n [get_collection_size $scan_diag_src]
+set scan_diag_dst_n [get_collection_size $scan_diag_dst]
+if { $scan_diag_src_n == 0 } {
+    post_message -type info "Ch442 SDC: scanout diagnostic CDC inactive in this profile (0 src -> no LPDDR scanout)"
+} else {
+    if { $scan_diag_src_n != 37 } { error "Ch442 SDC FATAL: scanout diag bundle source count $scan_diag_src_n != 37 (10+10+10+4+1+1+1; renamed/optimized? hold cut + net_delay would orphan)" }
+    if { $scan_diag_dst_n == 0 }  { error "Ch442 SDC FATAL: scanout diag bundle dest u_hps_bridge|scan_diag_*_q EMPTY (renamed? hold cut would orphan)" }
+    set_false_path -hold      -from $scan_diag_src -to $scan_diag_dst
+    set_max_skew              -from $scan_diag_src -to $scan_diag_dst 2.0
+    set_net_delay        -max -from $scan_diag_src -to $scan_diag_dst 2.0
+    post_message -type info "Ch442 SDC: scanout diag bundle hold-false-path + 2ns max_skew + 2ns net_delay ($scan_diag_src_n src -> $scan_diag_dst_n dst)"
+}
 post_message -type info "Ch357 SDC: osd_cfg refclk->design_clk synchronizer false-path ($osd_src_n src -> $osd_dst_n stage0 dst)"
 
 # Ch357 (Codex) — OSD MENU-STATE synchronizers (same quasi-static class as osd_cfg): cursor_row + osd_active cross