tui: keep history recall cursor at line end

[joshka-oai]

Summary

• keep cursor at end-of-line after Up/Down history recall
• allow continued history navigation when recalled text cursor is at start or end boundary
• add regression tests and document the history cursor contract in composer docs

Testing

• just fmt
• cargo test -p codex-tui --lib history_navigation_leaves_cursor_at_end_of_line
• cargo test -p codex-tui --lib should_handle_navigation_when_cursor_is_at_line_boundaries
• cargo test -p codex-tui (fails in existing integration test suite::no_panic_on_startup::malformed_rules_should_not_panic because target/debug/codex is not present in this environment)