Add quit_if_one_screen option for dynamic_paging mode

[keith-hall]

Adds a less -F-style auto-quit for dynamic_paging: when content fits on one screen and the end of output is signalled, the pager exits automatically and preserves the content on the terminal. Unlike set_run_no_overflow (static mode only), this supports buffering use cases where the total line count is unknown upfront.

New API

• Pager::set_quit_if_one_screen(bool) — enables the feature. When true, auto-quits with content preserved if it fits on screen once end-of-output is signalled. Otherwise the pager stays open for user scrolling/quitting.
• Pager::end_of_output() — explicitly signals end-of-output without dropping the Pager handle. Preferred when the caller needs to keep the handle alive (e.g. bat's use case).
• Drop-based signal — dropping all application-side Pager clones also fires the signal automatically via an AliveGuard RAII type.

Internals

• AliveGuard holds a channel sender and fires Command::CheckQuitIfOneScreen on drop; dynamic_paging receives its own independent Arc<AliveGuard> so only the caller's clones count toward the signal.
• PagerState::quit_if_one_screen: bool tracks the setting; the CheckQuitIfOneScreen handler restores the terminal, writes formatted lines to the main screen, then respects the configured ExitStrategy.

Usage

let pager = Pager::new();
pager.set_quit_if_one_screen(true)?;

let pager2 = pager.clone();
let t = std::thread::spawn(move || dynamic_paging(pager2));

pager.push_str("Hello\nWorld\n")?;

// Option A: explicit signal, handle stays alive
pager.end_of_output()?;

// Option B: implicit signal via drop
drop(pager);

t.join().unwrap()?;