M3: dead code purge + layer refs → semantic tokens

Dead code removed:
- 05-utilities: grid-2/card-grid/stats block (superseded by 08-layout)
- 02-semantic: dialog min-width:100% (conflicted with 03-components)
- 02-semantic: hgroup p dead font-family, redundant nested list margin
- All layers: [ai-disclosure] → [data-ai-disclosure] (bug fix)
- table.striped → [data-table="striped"] (classless alignment)

Ref cleanup: 12 --gray-N/--green-N/--blue-N replaced with semantic
tokens. 24 refs marked TODO (syntax highlighting needs --syntax-* aliases).

assets/css/layers/06-charts.css

@@ -571,7 +571,7 @@
   border-radius: 50%;
   background: conic-gradient(
     var(--color, var(--chart-color-1)) 0deg calc(var(--size, 0.5) * 360deg),
-    var(--surface-1, var(--gray-15)) 0deg
+    var(--surface-1, var(--surface)) 0deg
   );
   display: flex;
   align-items: center;