Introduces the 56px left icon rail specified in
docs/specs/2026-04-11-nexus-layout-overhaul.md §4.1. Four primary
destinations (Assistant, Studio, Projects, Settings) rendered as Lucide
icons with silver default + volt active state and a 2px volt bar on the
right edge of the active item. Destinations are company-prefixed except
Settings, which points at the global /instance/settings/general route.
The Studio icon also highlights on /convert because Phase 10 folds
ConvertPage into Studio as a workshop. The Projects icon is the umbrella
for all Phase 11 per-project-tab routes (issues, agents, routines,
goals, approvals, costs, activity, inbox, execution-workspaces).
The rail is not yet mounted in Layout.tsx — that happens in task 6.
Part of the Nexus v1.7 structural overhaul (Phase 8 of MIGRATION-PLAN.md
§8b). Companion tests cover all 4 destinations, active-state derivation,
and aria-current semantics.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
