Keep workspace chrome still when switching documents

Switching or creating a document cross-faded and slid the whole route
layer, so the header, tabs and Chat frame were drawn twice mid-switch.
The incoming route now loads unseen while the outgoing one stays on
screen until its document syncs (at most 500ms). Then only the document
body and transcript change, opacity only: out over 80ms, in over 150ms,
never overlapping. The header and Chat frame swap in place. Sibling
Document/Decisions tabs keep their directional slide.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>