fix(tui): keep unified exec summary on working line

[joshka-oai]

Problem

When unified-exec background sessions appear while the status indicator is visible, the bottom pane can grow by one row to show a dedicated footer line. That row insertion/removal makes the composer jump vertically and produces visible jitter/flicker during streaming turns.

Mental model

The bottom pane should expose one canonical background-exec summary string, but it should surface that string in only one place at a time:

• if the status indicator row is visible, show the summary inline on that row;
• if the status indicator row is hidden, show the summary as the standalone unified-exec footer row.

This keeps status information visible while preserving a stable pane height.

Non-goals

This change does not alter unified-exec lifecycle, process tracking, or /ps behavior. It does not redesign status text copy, spinner timing, or interrupt handling semantics.

Tradeoffs

Inlining the summary preserves layout stability and keeps interrupt affordances in a fixed location, but it reduces horizontal space for long status/detail text in narrow terminals. We accept that truncation risk in exchange for removing vertical jitter and keeping the composer anchored.

Architecture

UnifiedExecFooter remains the source of truth for background-process summary copy via summary_text(). BottomPane mirrors that text into StatusIndicatorWidget::update_inline_message() whenever process state changes or a status widget is created. Rendering enforces single-surface output: the standalone footer row is skipped while status is present, and the status row appends the summary after the elapsed/interrupt segment.

Documentation pass

Added non-functional docs/comments that make the new invariant explicit:

• status row owns inline summary when present;
• unified-exec footer row renders only when status row is absent;
• summary ordering keeps elapsed/interrupt affordance in a stable position.

Observability

No new telemetry or logs are introduced. The behavior is traceable through:

• BottomPane::set_unified_exec_processes() for state updates,
• BottomPane::sync_status_inline_message() for status-row synchronization,
• StatusIndicatorWidget::render() for final inline ordering.

Tests

• Added bottom_pane::tests::unified_exec_summary_does_not_increase_height_when_status_visible to lock the no-height-growth invariant.
• Updated the unified-exec status restoration snapshot to match inline rendering order.
• Validated with:
  • just fmt
  • cargo test -p codex-tui --lib