Per design feedback, drop the top-tabs + single-ribbon model. The mobile UI
is now one vertical stack of all panes in a fixed order (Outliner, Asset
Library, Stage, Timeline, Piano Roll, Virtual Piano, Node/Instrument, Script
editor) with a window of 2-3 consecutive panes visible at a time.
Dragging the dividers between panes and the top/bottom screen edges grows,
shrinks, or slides the window via a small state machine (R1-R6 in stack.rs),
with continuous interpolation during the drag and snap-on-release. The
Node/Instrument band toggles between the node editor and the preset browser.
The transport stays pinned as the bottom floor.
Each visible band reuses PaneInstance::render_content full-bleed (surface.rs).
Removes topbar.rs and ribbon.rs; rewrites mod.rs state (window_top,
window_count, show_instruments, drag); adds stack.rs.
79f063d00c