feat: overlap renders stacked author marks (insert + delete) in both panes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

media/preview.css

@@ -35,6 +35,14 @@ del { text-decoration: line-through; opacity: 0.7; }
 .cw-del-claude { background: rgba(188,140,255,0.13); text-decoration: line-through; text-decoration-color: #bc8cff; }
 .cw-del-none   { background: var(--vscode-diffEditor-removedTextBackground, rgba(248,81,73,0.11)); text-decoration: line-through; opacity: 0.7; }
 
+/* Task 8 — overlap: if a future render path nests cw-ins-{A} inside cw-del-{B},
+   both marks must show. cw-ins-* sets text-decoration:none which would otherwise
+   suppress the parent del's strikethrough. `inherit` restores the parent's
+   line-through while the child's border-bottom underline is unaffected.
+   The engine currently emits SIBLING ins/del (never nested) so this rule is a
+   forward defensive guarantee only. See task-8-report.md for details. */
+.cw-del-claude .cw-ins-human, .cw-del-human .cw-ins-claude { text-decoration: inherit; }
+
 /* whole-block ops: an added block is an insertion (its author runs are emitted as
    cw-ins-* via colorByAuthor with kind="ins"); a standalone removed block is neutral
    struck (adjacency heuristic fallback) */