book(I2.1): GFM task lists + nested lists in pagination

marked switches to gfm:true everywhere (lexer + parser), unlocking:

- Task lists — `- [ ] todo` / `- [x] done` parse as list items with
  task:true + checked:bool. The pagination engine emits ☐ / ☑ as
  the marker prefix (replacing the bullet) so paginated and preview
  output both render the checkbox state.
- Nested lists — list items whose tokens contain an inner list
  recurse with a 3-space-per-level indent. Capped at depth 6 so
  pathological input can't blow the stack.
- Autolinks + strikethrough — parsed by marked into the HTML
  preview pane (the pagination engine flattens to plain text so
  strike doesn't visually render in paginated tiles, but the
  inline preview shows it).

Tables get a placeholder row in v1 — "[Table — N cols × M rows: …]"
with a warning surfaced through pag.warnings. Full column layout
ships in I2.2 once the renderer side (DOM + canvas + PDF emission)
gains a per-cell branch.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/index.html

@@ -9897,7 +9897,7 @@
         padding,
       };
 
-      const tokens = window.marked.lexer(md);
+      const tokens = window.marked.lexer(md, { gfm: true, breaks: false });
 
       // One shared Canvas2D context for text-width measurement.
       // Sized at 96 px/inch so widths come out in pixels and we
@@ -9989,16 +9989,38 @@
           // no wrapping (the user controls line lengths).
           lines = String(token.text || '').split('\n');
         } else if (token.type === 'list') {
-          // Flatten list items into "• text" lines. Each item
-          // wraps using the list style.
-          const items = Array.isArray(token.items) ? token.items : [];
-          const ordered = !!token.ordered;
-          items.forEach((item, i) => {
-            const marker = ordered ? `${i + 1}. ` : '• ';
-            const text = String(item.text || '');
-            const itemLines = wrapTextIntoLines(marker + text, availW);
-            lines = lines.concat(itemLines);
-          });
+          // Flatten list items into prefixed lines. Task items get
+          // ☐ / ☑ instead of the bullet; nested lists recurse with
+          // a 3-space-per-level indent. Cap at 6 levels so a
+          // pathological input can't blow the stack.
+          const renderList = (listToken, depth) => {
+            const items = Array.isArray(listToken.items) ? listToken.items : [];
+            const ordered = !!listToken.ordered;
+            items.forEach((item, i) => {
+              const indent = '   '.repeat(Math.min(depth, 6));
+              const marker = item.task
+                ? (item.checked ? '☑ ' : '☐ ')
+                : (ordered ? `${i + 1}. ` : '• ');
+              const text = String(item.text || '');
+              const itemLines = wrapTextIntoLines(indent + marker + text, availW);
+              lines = lines.concat(itemLines);
+              if (depth < 6) {
+                for (const inner of (item.tokens || [])) {
+                  if (inner?.type === 'list') renderList(inner, depth + 1);
+                }
+              }
+            });
+          };
+          renderList(token, 0);
+        } else if (token.type === 'table') {
+          // Tables get a placeholder row in v1 — proper column
+          // layout lands in I2.2.
+          const headerLabels = (token.header || [])
+            .map(c => (c?.text || c?.tokens?.[0]?.text || c?.raw || '')).filter(Boolean);
+          const rowCount = Array.isArray(token.rows) ? token.rows.length : 0;
+          const summary = `[Table — ${headerLabels.length} cols × ${rowCount} rows: ${headerLabels.join(' · ')}]`;
+          lines = wrapTextIntoLines(summary, availW);
+          result.warnings.push(`Tables render as a placeholder in v1 — full layout in I2.2.`);
         } else if (token.type === 'blockquote') {
           // marked emits blockquote with inner tokens; concatenate
           // their plain text + wrap.
@@ -11529,7 +11551,7 @@
           mangle: false,
           headerIds: false,
           breaks: false,
-          gfm: false,
+          gfm: true,
         });
         // Belt-and-braces: drop any embedded <script> just in case.
         return String(html).replace(/<script[\s\S]*?<\/script>/gi, '');