The MobileScoreStrip lands: sticky under the app bar, one pill per
panel with the round-synced colored share, horizontally scrollable (the
only horizontal scroller, verified at 320 px), tapping scrolls to the
panel (instant under reduced motion, scroll-margin clears the sticky
stack).
Touch targets reach 44 px on mobile: player buttons, panel kebabs, and
toolbar buttons; Export collapses to its icon so the toolbar stays one
row down to 320 px.
