feat: add markdown view component

docs/CHECKLIST.md

@@ -9,15 +9,13 @@ is the first downstream consumer; its screens drive the order.
 
 ## Next
 
-- [ ] **Markdown view** (M16, `editor`) — only if a consumer confirms.
-    - [ ] API sketch
-    - [ ] Implementation
-    - [ ] Tests
-    - [ ] Gallery panel
-    - [ ] Review
-
 ## Queue — to follow
 
+- [ ] **Markdown view follow-ups** — streaming `append` with a coalesced
+      background parse, text selection with copy, per-block a11y roles,
+      GFM alerts + table column alignment, per-fence highlight reuse,
+      `soft_break_as_hard_break`; ranked notes in
+      [markdown-view-comparison.md](markdown-view-comparison.md).
 - [ ] **Editor follow-ups** — language detection by file path, meta line
       overlay, per-view config overrides (line height, gutter visibility).
 - [ ] **Pickers** — Calendar / DatePicker, TimeField, ColorPicker (decide at
@@ -76,8 +74,10 @@ is the first downstream consumer; its screens drive the order.
       incremental line re-highlight
 - [x] Diff view — unified rows via `similar`, highlighted with the code
       view's palette; change washes from the ramps, `scroll_to_first_change`
-- [x] Diff view — unified rows via `similar`, highlighted with the code
-      view's palette; change washes from the ramps, `scroll_to_first_change`
+- [x] Markdown view — `pulldown-cmark` blocks rendered as formatted text:
+      headings, emphasis runs, ordered/task lists, quotes, fenced code
+      highlighted with the code view's palette, tables, clickable links;
+      virtualized variable-height rows over gpui's `list`
 
 ### layout (`crates/layout`)