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>

AGENTS.md

@@ -13,7 +13,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 - Web views: `wry` 0.57 (Apache-2.0 OR MIT, default features off, macOS only) lays a child WKWebView over the window.
 - Terminal: `alacritty_terminal` 0.26 (Apache-2.0, default features off, native only) for the grid, its parser and the pseudo-terminal; `futures` carries its events. `vte` 0.15 (Apache-2.0 OR MIT), the parser it re-exports, reads `AnsiText`'s codes alone, so colored output builds for wasm32.
 - Diffs: `similar` 3.2 (Apache-2.0) for line and word diffs and three-way merges; its `unicode` feature splits words at punctuation.
-- Markdown: `pulldown-cmark` 0.13 (MIT, default features off) for CommonMark with tables, tasks, strikethrough, footnotes and math; `BlockData::from_markdown` adds GitHub's alerts, a callout's form, and reads each field's inline Markdown from the source by offset, as written.
+- Markdown: `markdown` 1.0.0 (markdown-rs, MIT) reads GitHub's flavor and math into a syntax tree with byte positions for `MarkdownRenderer`, `MarkdownEditor`, the highlights, block ranges and outline. `BlockData::from_markdown` and its export still read with `pulldown-cmark` 0.13 (MIT, default features off); it adds GitHub's alerts, a callout's form, and reads each field's inline Markdown from the source by offset, as written.
 - Pictures: `image` 0.25 (MIT OR Apache-2.0), the crate gpui decodes with, default features off; it turns decoded frames and rims map tiles.
 - Maps: Natural Earth's 110m countries (public domain). `assets/maps/countries.geojson`, trimmed by `scripts/countries.py` to an ISO code, a name and positions to 0.01°, backs `maps::WorldMap`; the gallery's tiles are cut from the same file, z0 to z3, by `scripts/tiles.py` with Pillow.
 - Web: on wasm32, `web-time` 1.1 (MIT OR Apache-2.0) gives the `Instant` gpui's executor returns, std's own natively; jiff takes `js`, rust-embed `debug-embed`, similar `wasm32_web_time`. The gallery's web entry takes `gpui_web`, `wasm-bindgen`, `js-sys` and `web-sys` as wasm-only dev-dependencies, and a Hebrew cut of Noto Sans Hebrew 2.003 (OFL, `examples/gallery/fonts`): gpui_web asks the browser for emoji and CJK alone. The `wasm-bindgen` CLI at Cargo.lock's version and binaryen's `wasm-opt` finish the bundle.