feat(ui-lab): preview What's New entries via ?changelog=

Add a UI-Lab-only (mode === "ui", dead-code-eliminated from real builds)
URL override for the What's New modal: ?changelog=unreleased shows the
in-progress notes before they ship, ?changelog=small serves a synthetic
hotfix-sized entry to exercise the compact layout (including a no-lead
bullet), and ?changelog=<version> previews any released entry. Documented
in docs/ui-lab.md alongside the ?scenario= table.

docs/ui-lab.md

@@ -55,6 +55,20 @@ http://127.0.0.1:1422/?scenario=errors
 http://127.0.0.1:1422/?scenario=huge
 ```
 
+## Changelog Previews
+
+The What's New modal adapts its layout to release size (compact single column
+for small releases, a two-pane section rail for large ones). UI Lab reads
+`?changelog=` to override which entry the modal shows:
+
+| URL | Purpose |
+| --- | --- |
+| `/?changelog=unreleased` | The in-progress `[Unreleased]` notes — large release, rail layout |
+| `/?changelog=small` | Synthetic hotfix-sized entry — compact single-column layout |
+| `/?changelog=0.1.1` | Any specific released version |
+
+Open the modal via the demo panel: `Ctrl+Shift+D` → Open "What's new" modal.
+
 ## How It Works
 
 `src/main.tsx` bootstraps the app asynchronously. In `ui` mode it imports