fix(terminal): batch visible output repaints

[Jinwoo-H]

Summary

• coalesce visible terminal output for one frame before writing to xterm
• flush pending visible output before forwarding terminal input
• extend terminal output scheduler tests for foreground batching and explicit flushes

Why

Codex-style TUIs repaint the prompt/status area by moving the terminal cursor through intermediate positions. Writing every PTY chunk to xterm immediately can expose those positions as cursor flicker on Windows. Batching visible output to one frame keeps repaint bursts together while preserving explicit flush points for input and replay-sensitive paths.

Test plan

• pnpm vitest run --config config/vitest.config.ts src/renderer/src/lib/pane-manager/pane-terminal-output-scheduler.test.ts src/renderer/src/components/terminal-pane/use-terminal-pane-lifecycle.test.ts
• pnpm exec oxlint --format github src/renderer/src/lib/pane-manager/pane-terminal-output-scheduler.ts src/renderer/src/lib/pane-manager/pane-terminal-output-scheduler.test.ts src/renderer/src/components/terminal-pane/pty-connection.ts
• pnpm run typecheck

Note: local Windows full pnpm test is blocked by Node 25/native module mismatch and Windows named pipe behavior; CI runs Node 24 on Linux.