fix(timeline): drop the test's any and refresh the checklist's line refs

Two findings from the automated review on aa8d3bb:

- the mocked `tl` was cast to `any` behind a biome-ignore, which AGENTS.md rules
  out ("don't add new `any`"). Cast through `unknown` to the real
  `ReturnType<typeof useTimeline>` instead: the prop keeps its type and the
  suppression goes away.
- the "add a region kind" checklist pointed at pre-PR line numbers in
  V4Timeline.tsx, and this branch moved them by ~130 lines. Recomputed against
  the current file: :461-509 for the pill call site, :1478-1488 for the lane
  render block, :332 for the `kind` union. (They were already drifting before
  this branch — check-docs does not verify line numbers.)

The third finding, `currentColor` → `currentcolor` for stylelint's
value-keyword-case, does not apply here: the repo has no stylelint at all (CI's
Lint job is `biome check`, green on both spellings), and the convention in
website/src/css/custom.css is `currentColor`. Changing it would leave the only
lowercase spelling in the codebase.

Author: EtienneLescot

src/components/ai-edition/v4/V4Timeline.geometry.test.tsx

@@ -13,6 +13,7 @@ vi.mock("@/contexts/I18nContext", () => ({
 }));
 vi.mock("sonner", () => ({ toast: { error: vi.fn(), info: vi.fn(), success: vi.fn() } }));
 
+import type { useTimeline } from "@/lib/ai-edition/store/useTimeline";
 import { V4Timeline } from "./V4Timeline";
 
 beforeAll(() => {
@@ -86,8 +87,9 @@ function renderTimeline(
 	};
 	render(
 		<V4Timeline
-			// biome-ignore lint/suspicious/noExplicitAny: a partial timeline API is enough to draw pills
-			tl={tl as any}
+			// Only the members the lanes and the clip row read are mocked; the prop
+			// stays typed as the real API rather than widened to `any` (AGENTS.md).
+			tl={tl as unknown as ReturnType<typeof useTimeline>}
 			setCurrentTime={vi.fn()}
 			playing={false}
 			onTogglePlay={vi.fn()}

technical-documentation/architecture/editor-shell.md

@@ -129,9 +129,9 @@ switches on it is checked below. Each path has been verified on this branch.
    (`document/timeline.ts:591`) for batch / single deletes.
 4. **Lane in `V4Timeline`** — `src/components/ai-edition/v4/V4Timeline.tsx`.
    Compute the pills at the same call site as the four existing lanes
-   (`coalesceRegionsForRuler(tl.xxxRegions).map(...)` near `:321-347`), render
+   (`coalesceRegionsForRuler(tl.xxxRegions).map(...)` near `:461-509`), render
    them through `renderPills` inside a `<div className={styles.tlLane}>`
-   block (`:1244-1252`), and extend the `kind` union at `:203` so drag,
+   block (`:1478-1488`), and extend the `kind` union at `:332` so drag,
    resize, and delete handler switches route correctly.
 
    **Coordinates on the timeline canvas obey one rule**: position and size are