feat(md-print): add A4/A5 multi-column print layout

Add an md-print element that paginates markdown into A4 landscape sheets holding two A5 pages, each with configurable columns. The preview modal exports through the browser print dialog.

Paragraphs, lists, and tables split at word, item, and row boundaries; headings are kept with following content.

docs/reference.md

@@ -48,6 +48,7 @@ Nest with more colons (`::::`, `:::::`).
 | `:md-dice` | dice formula | `key` |
 | `:md-table` | CSV path or inline id | `roll`, `remix` |
 | `:md-deck` | CSV path | `size`, `grid`, `layers` |
+| `:md-print` | markdown path, optional #section | `columns`, `margin`, `gap`, `font`, `title`, `pageNumbers`, `cutMarks` |
 | `:md-link` | path, optional #section | — |
 | `:md-embed` | path, optional #section | `headingBase` |
 | `:md-bg` | image path or CSS color | `fit` |
@@ -111,6 +112,31 @@ Prefer the `yaml role=tag` form — it is the only form that supports template l
 :md-deck[./cards.csv]{size="63x88" grid="5x5" layers="title:1,1-5,1f8 body:1,5-5,8f3"}
 ```
 
+### `:md-print`
+
+Paginates a markdown document into A4 landscape sheets for printing.
+
+Each sheet holds two A5 pages side by side, and each A5 page flows the markdown into `columns` columns. `margin` and `gap` are in millimetres, `font` is in points. `pageNumbers` adds an `n / total` footer and `cutMarks` draws the centre cut line. The preview modal exports through the browser's print dialog (choose A4 landscape, no margins, 100% scale).
+
+- Body: markdown path, optional #section
+- Attributes: `columns`, `margin`, `gap`, `font`, `title`, `pageNumbers`, `cutMarks`
+
+```markdown
+:md-print[./rules.md]{columns="2" margin="10" gap="5" font="9"}
+```
+
+````markdown
+```yaml role=tag
+tag: md-print
+body: ./rules.md
+columns: 2
+margin: 10
+gap: 5
+font: 9
+title: 规则书
+```
+````
+
 ### `:md-link`
 
 Clickable link that expands the target article inline.