documents: exports read through MarkdownReader, one fresh stack per area; reading helpers close again

The writer in ElyMark kept its own copy of the inline reading, which drifted
from the drawing. Reader goes public as MarkdownReader with block, inline,
note and label, and its blocks as MarkdownBlock. Shortcodes, InlineTag,
inline_tag and html_block_text close back to the tree; headings(text) goes.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/mod.rs

@@ -58,8 +58,7 @@ pub use thumbs::PageThumbnailList;
 pub use todo::{Checklist, TodoItem};
 pub use toolbar::FixedFormatToolbar;
 pub use tree::{
-    AlertKind, Inline, InlineTag, Shortcodes, Style, alert, headings, html_block_text, inline_tag,
-    is_toc,
+    AlertKind, Inline, Node as MarkdownBlock, Reader as MarkdownReader, Style, alert, is_toc,
 };
 pub(crate) use viewer::step as zoom_step;
 pub use viewer::{DocPage, DocumentViewer, PageHit, PageNote};