fix(cli): contrast gate judges only readable content; examples pass check

Three filters keep the escalated contrast gate honest, each surfaced by
running check across the registry examples:
- data-layout-ignore (the layout audit's existing decorative opt-out)
  now also excludes set-dressing text from the contrast audit — one
  vocabulary for 'not copy a viewer must read'.
- Text that has (nearly) left the canvas is skipped: sampling a clamped
  off-canvas box reads border pixels and produced the classic false
  white-on-white (a cursor exiting the frame).
- Contrast failures follow the same persistence rule as layout findings:
  observed at a single sample of a multi-sample sweep demotes to
  warning; held failures gate.

Example fixes the sweep exposed: motion-blur's 21 deliberately-dim rail
labels are marked decorative (its vivid labels pass on their own);
nyt-graph's subtitle and source-note adopt the gate's suggested
compliant gray; decision-tree's declared duration drops 15s to 10s —
its content ends at ~9.5s and every render shipped a blank white tail.

registry/examples/nyt-graph/compositions/nyt-chart.html

@@ -95,7 +95,7 @@
 
       [data-composition-id="nyt-chart"] .subtitle {
         font-size: 16px;
-        color: #d4d4d4;
+        color: #737373;
         margin: 8px 0 16px 0;
         clip-path: inset(0 100% 0 0);
         white-space: nowrap;
@@ -141,7 +141,7 @@
         bottom: 0;
         left: 0;
         font-size: 11px;
-        color: #999;
+        color: #737373;
         opacity: 0;
       }