terminal/tmux: test output state across pane moves

Add regression coverage for the persistent VT stream introduced in #14676. The existing output-state tests cover split CSI input, but do not exercise an unfinished OSC sequence while the pane map is rebuilt.

The new tests start an OSC title sequence, rebuild the map while retaining the pane, and then either complete the sequence or remove the pane before completion. They check the resulting title and the removal/cleanup path.

This is a test-only change; it does not add another parser implementation.

Related to #14676 and #1935.

AI disclosure

I used OpenAI Codex for code walkthroughs, review, and drafting the test examples.