Timeline: deep-link / share a selected commit #176

Timeline row selection is local state — a URL can't point at a specific commit. Two shapes considered (during Timeline core-page planning) for making a selection shareable:

1. URL search param, reusing the Graph page's existing vocabulary: select_node_id whose value is a metadata-file path (the Graph page already navigates with exactly this — see useViewInGraph). Timeline and Graph links would become symmetric, and validateSearch covers it. Needs navigate({ replace: true }) on every selection so arrow-key browsing doesn't spam history, and the page must subscribe to search with a select so selection changes don't re-render the data layer.
2. A share button on the details panel that copies a URL with the param — same plumbing, but the address bar stays clean during normal browsing.

Also requires scrolling the selected row into view on load, and silently ignoring a param that points outside the loaded range.

Deferred from the core page ticket because the URL-key design deserves its own decision.