feat: add selection, active line, and incremental highlighting to code

view

docs/CHECKLIST.md

@@ -9,15 +9,21 @@ is the first downstream consumer; its screens drive the order.
 
 ## Next
 
-- [ ] **Diff view** (M15, `editor`).
-    - [ ] API sketch
-    - [ ] Implementation
-    - [ ] Tests
-    - [ ] Gallery panel
+- [ ] **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
     - [ ] 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).