Shared footer-band geometry for the single-active secondary overlays (#2281).

The Go compositor historically footer-appended one secondary overlay at the bottom of the screen, sizing its height with maxOverlayHeight() (go/tui/internal/ui/model.go: min(max(height/3, 4), 12)) and spanning the full terminal width. These eight surfaces (float popup, agent context, tool manager, extension panel, observatory, edit timeline, notifications, extension overlay) are identical footer-band shapes, so the BEAM owns one rect helper for all of them instead of eight copies.

rect/2 ports that exact clamp. content_height is the surface's content line count (BEAM-derivable from the chrome/UI models) or :max for a surface whose wrapped line count the BEAM cannot know. The rect is bottom-anchored: it occupies the lowest height rows of the terminal and the full width, the same band the frontend appended into, so the rect's bottom edge sits where the old footer-append put the overlay (directly above the minibuffer).

Only one overlay shows at a time in that band (single-active model, #2268 AC-4). For the count-derivable surfaces (notifications, observatory, edit_timeline, extension_overlay) the caller passes a real content_height so the band shrinks to the content and the overlay hugs the screen bottom. For the wrap-dependent surfaces (float_popup, extension_panel, and the dormant agent_context/tool_manager) the caller passes :max (the clamp ceiling); the frontend bottom-aligns the rendered content inside that band, so a short overlay still hugs the bottom and the residual phantom rows sit ABOVE it (see MingaEditor.Layout.FooterOverlays). Either way the rect only needs to CONTAIN the overlay so clicks over it are hit-tested, not pixel-match it.