feat: apply declarative render rule across components

docs/CHECKLIST.md

@@ -9,12 +9,16 @@ is the first downstream consumer; its screens drive the order.
 
 ## Next
 
-- [ ] **Declarative render audit** — run the AGENTS.md render rule over the
-      other landed components: replace `let mut` element variables reassigned
-      under `if` / `if let` with one chained expression using `.when(...)` /
-      `.when_some(...)` / `.map(...)`. The sidebar row is the model; sweep
-      `crates/core`, `crates/data`, `crates/editor`, `crates/layout`, and
-      re-run tests plus a gallery pass after each crate.
+- [x] **Declarative render audit** — the AGENTS.md render rule swept the
+      landed components: `let mut` element variables reassigned under `if` /
+      `if let` now build one chained expression with `.when(...)` /
+      `.when_some(...)` / `.map(...)`. Converted: Tabs strip root and tab,
+      Menu / DropdownMenu / Popover / Combobox wrappers, ScrollArea pane and
+      scrollbars, Select rows, Dock tab entries, Resizable panes and dividers,
+      the Markdown text / code / table rows, and the Diff gutter cell.
+      `MenuItem` and `Tab` adopted `FluentBuilder` so their builders chain
+      too; data accumulators (`Vec`/`String` builders, style clones) stay
+      imperative — they are state, not element refinement.
 
 ## Queue — to follow