Adds the approved framing for the bottom-panel arc: a buffer displayed
in a fixed-height window pinned to the bottom of the frame, targeted by
policy rather than by stealing the selected window, plus the missing
display-buffer/window-parameter concept underneath it.
Revision 4 follows three review rounds, an integration review, and a
landed-state audit against GPU initial target (#148, protocol v20) and
folding Stage 2 (#149).
Amended before branching by the pre-implementation dependency
verification recorded in section 0.6:
- the folding dependency is cleared and re-verified against canonical
main at ddaa80d (nothing in flight, folding Stage 2 acceptance 48/48
green, every borrowed anchor reproducing, and folding's only window.rs
edit confined to one 22-line hunk that leaves the layout functions
pre-folding code);
- R5-B1: Layout::compute has TWO production callers, not one. The
second, the peer-presence overlay pass in src/overlay_paint.rs, builds
its own text-area rect from active_layout() and never routes through
window_placements, so the planned compute(area, fixed) signature change
would otherwise leave every peer cursor painted at its no-panel row.
Corrected in section 1.1 and Q#BP2, pinned by acceptance 1, and the
fixed map is now specified to come from one shared helper rather than
being assembled per call site.
Stage 1 (window placement + TUI side windows) changes no wire shape.
