feat: add diff view component

docs/CHECKLIST.md

@@ -9,22 +9,17 @@ is the first downstream consumer; its screens drive the order.
 
 ## Next
 
-- [ ] **Code view interactions** (M15, `editor`) — active line, text
-      selection with copy, incremental line re-highlight.
-    - [x] API sketch
-    - [x] Implementation
-    - [x] Tests
-    - [x] Gallery panel
+- [ ] **Markdown view** (M16, `editor`) — only if a consumer confirms.
+    - [ ] API sketch
+    - [ ] Implementation
+    - [ ] Tests
+    - [ ] Gallery panel
     - [ ] Review
 
 ## Queue — to follow
 
-- [ ] **Diff view** (`editor`) — implemented, then reverted to
-      `stash@{0}` ("pre-revert: diff view + gutter tweaks"); restore with
-      `git stash pop` and re-land after the code view interactions settle.
 - [ ] **Editor follow-ups** — language detection by file path, meta line
       overlay, per-view config overrides (line height, gutter visibility).
-- [ ] **Markdown view** (M16, `editor`) — only if a consumer confirms.
 - [ ] **Pickers** — Calendar / DatePicker, TimeField, ColorPicker (decide at
       unit time).
 - [ ] **Layout follow-ups** — persistence, pane zoom, bottom dock,
@@ -77,6 +72,12 @@ is the first downstream consumer; its screens drive the order.
 - [x] Code view — tree-sitter highlighted, virtualized rows with a gutter
       and the overlay scrollbar; per-language grammar features, roles map to
       the theme's semantic palette at render
+- [x] Code view interactions — active line, text selection with copy,
+      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`
 
 ### layout (`crates/layout`)