New components: `SecondaryNav` and `NavigationLayout` for contextual secondary navigation

## What is being proposed?

Two new components, built on top of the existing `SidebarNav`:

- **`SecondaryNav`**: a second, contextual navigation panel (grouped items, optional one-level nesting, optional search, optional title/back header) that sits alongside the primary sidebar. It renders as a fixed side panel on desktop and a temporary overlay drawer on mobile.
- **`NavigationLayout`**: a composing layout component that owns `SidebarNav` + `SecondaryNav` + main content together, and handles the responsive coordination between the two panels (e.g. only one temporary drawer visible at a time on mobile, back-navigation between them).

## Why is this needed?

`SidebarNav` alone only models a single, flat/nested primary navigation tree. Several Diamond apps need a second level of navigation that's contextual to what's selected in the primary sidebar (e.g. picking Experiments in the sidebar, then navigating between each experiment in a secondary panel), something like `SidebarNav`'s pattern, but for a scoped set of items that changes with context.

Without a shared component, each consuming app would re-implement this panel-plus-drawer pattern, including the fiddly responsive behaviour (mobile drawer stacking, back button wiring, history/popstate handling)
that `NavigationLayout` centralises.

## What will change?

- Introduces two new components: `SecondaryNav` and `NavigationLayout` (both already implemented and exported from `src/index.ts` on this branch).
- No changes to existing component props/behaviour, other than a small addition to `SidebarNav` (see below) needed for `NavigationLayout` to drive it.
- New shared type: `LinkProps` in `src/components/navigation/types.ts`, reused by `SecondaryNav`'s item definitions.

## Interface changes (if any)

```tsx
// Standalone SecondaryNav

// Composed with SidebarNav via NavigationLayout

{children}

```

## Breaking change?

- [ ] Yes
- [X] No

## Next steps
A maintainer will review this issue.
If accepted, it will be marked as `accepted` and a PR may then be opened.

Start by reviewing the linked pull request and the exports in src/index.ts, then read the shared LinkProps definition in src/components/navigation/types.ts. Compare the implemented SecondaryNav and NavigationLayout with the proposed interfaces and responsive behavior; done means the requested components and SidebarNav integration are reviewed and complete.

Written by the indexing model from the issue text.