Optimize GridBash runtime hot paths

[jasonsuhari]

Problem

GridBash can run up to 100 PTY-backed panes, but several runtime hot paths still do repeated work under load: exit polling every loop, per-render footer scanning, linear pane lookup for PTY events, immediate redraw pressure from noisy output, and full pane row rebuilding.

Requested behavior

Reduce avoidable runtime overhead while preserving the current v1 single-process/session-scoped behavior.

Acceptance checks

• Throttle child exit polling instead of checking every pane every loop.
• Cache conversation footer summaries so normal renders do not rescan terminal screens.
• Coalesce PTY output bursts before parsing/rendering where practical.
• Avoid linear pane lookup for PTY events.
• Add a dirty/cached render path for unchanged pane screens.
• Existing tests pass.