tabs: the strip can stand on its side (tab_bar_side left/right)

A sidebar lists one tab per row down the chosen edge, tab_bar_width
columns wide; the panes trade the strip's row for the sidebar's columns
in tab_area(), the one place that arithmetic lives. The per-tab cell
drawing is extracted from draw_tab_strip (label, rename editor, bell,
hover, drop states, hit) and shared, so every mouse verb -- click,
double-click rename, drag to a tab, drop on + -- works in both shapes
because the hits move with the cells.

The strip's indicators (pane count, prefix badge) take the sidebar's
bottom rows, reserved first under the same budgeting rule the strip
applies from the right. The status line below keeps the full width. A
terminal too narrow to give up the columns falls back to top, re-derived
per frame, so a resize moves the bar without anyone storing state.

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