Build nav region builder for plan site

Convert the existing buildNavigation() output (NavGroup[]) into a renderable tag tree that can be passed as a layout region to renderFullPage().

Currently the sidebar HTML is hand-rolled in the HTML shell template. This work item replaces that with a buildNavRegion() function that produces serialized tags with BEM classes matching the existing .rf-plan-sidebar__* selectors.

Acceptance Criteria

buildNavRegion(groups, activePath) function exists in @refrakt-md/plan
Output is a serialized tag tree (plain {$$mdtype:'Tag'} objects)
Tags use correct BEM classes: .rf-plan-sidebar, .rf-plan-sidebar__group, .rf-plan-sidebar__link, .rf-plan-sidebar__link--active
Entity links include data-status, data-id attributes for future filtering (SPEC-015)
Existing sidebar visual appearance is preserved
Unit tests verify the tag tree structure

Approach

Create buildNavRegion() alongside the existing buildNavigation() in render-pipeline.ts. It walks the NavGroup[] array and emits tags with appropriate BEM classes and data attributes. The output plugs directly into LayoutPageData.regions.nav.