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- 05-utilities: grid-2/card-grid/stats block (superseded by 08-layout)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/03-components.css

@@ -701,7 +701,7 @@ dialog:not([open]) {
 
 /* Backdrop (modal mode only — showModal()) */
 dialog::backdrop {
-  background: color-mix(in oklch, var(--gray-15) 70%, transparent);
+  background: var(--modal-overlay);
   backdrop-filter: blur(4px);
   -webkit-backdrop-filter: blur(4px);
 }