Implements the §9.7 shell rulings (issue #173): "/" renders the library
and the board route is gone — every former board target (Esc, cleared
search, detail parent routes, the not-found fallback) lands on the
library. The signal chain moves into /settings as its first section,
lamps still driven by the health poll; the master lamp stays on the
rail. The wordmark is now a real anchor that navigates home.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
