fix terminal panel IME activation on Windows

[peters]

Summary

• emit egui IME output for focused terminal panels so the native window backend enables IME and positions the candidate UI
• track terminal-local IME composition state and filter IME-incompatible key events while composition is active
• add focused unit coverage for terminal IME event ordering and cursor rect calculation

Root Cause

The terminal widget handled committed IME text, but it never published PlatformOutput.ime while focused. With egui-winit, that leaves set_ime_allowed(false) in effect, so Windows does not activate IME for the terminal panel.

Impact

This gives terminal panels the same IME activation signal that egui text widgets already provide, which should allow Windows IME candidate windows to appear and commit text into the PTY.

Validation

• cargo fmt --all -- --check
• ./scripts/check-maintainability.sh
• RUSTFLAGS='-D warnings' cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic

Refs #154