Markdown: MarkdownRenderer, MarkdownEditor, highlights and block ranges read with markdown-rs

The tree comes from markdown::to_mdast with GitHub's flavor and math.
References resolve against their definitions, an image splits its
paragraph, a one-line $$ formula stands as a block. BlockEditor's
import and export keep pulldown-cmark.

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

Cargo.toml

@@ -26,6 +26,7 @@ isocountry = "0.3"
 isolang = { version = "2.4", features = ["local_names", "list_languages"] }
 jiff = "0.2"
 log = "0.4"
+markdown = "=1.0.0"
 pulldown-cmark = { version = "0.13", default-features = false }
 qrcode = { version = "0.14", default-features = false }
 regex = "1"