Support multiple bottom airline/statusline rows in the CLI

Summary

Codex CLI should support multiple bottom airline/statusline rows, not just a single footer strip.

Problem

A single bottom row becomes too constrained once the statusline starts carrying more than a few small fields. This makes richer airline/status widgets compete horizontally for a very limited strip of space.

That constraint weakens many otherwise-useful ideas:

• edited file stacks
• token/context breakdowns
• agent/thread badges
• richer mode/status surfaces
• animated widgets

Requested behavior

• allow more than one bottom airline/statusline row in the CLI
• let rows be configured independently
• keep the layout stable so row count does not feel like footer jitter
• support a split where one row carries compact essentials and another row carries richer/contextual widgets

Why this matters

If Codex gets a custom statusline widget API, one-row-only layout will still bottleneck expressiveness. Multi-row support is the layout companion that makes richer widgets actually practical.

Example direction

• row 1: compact always-on essentials (model, cwd, branch, fast mode, etc.)
• row 2: richer widgets (edited file stack, token/context composition, active agents, animated progress/status)

Success criteria

• users can configure more than one bottom airline/statusline row
• richer widgets no longer have to fight for one cramped horizontal strip
• the footer remains visually stable and readable

Related context

Companion proposal for extensibility:

• #14043

Multi-row layout and custom widgets fit together naturally: the widget API expands what can be rendered, and multi-row support gives those widgets enough space to be useful.