feat(oc-docs): page-based routing with navigation

[sundram-bruno]

Page-based navigation for the docs renderer (BRU-3188): replaces the single long scroll with one page per item, each at a stable, shareable slug URL.

What's included

• Routing module (routing/): slug (path-based slugify + sibling dedupe), navModel (ordered DFS of the collection honouring folder hierarchy + seq), resolve/hooks (active entry + prev/next derived from the URL).
• AppShell: composes Topbar + sidebar + routed page content (+ playground drawer).
• PageRouter: renders one page by type — Overview, Request, Folder, Script, Environments.
• PrevNext footer: hierarchy/seq-ordered pagination — Fira Code method labels, edge chevrons, ellipsis on long names, responsive padding; aligned to the 1200px content gutter.
• Sidebar wired to the route: active item derived from the URL + expand-only auto-reveal of the active node's ancestors.
• Routing state lives in the existing store/slices/docs (expandFolders) — no parallel store.

routing-demo1.mov