Remove AliveGuard from dynamic paging quit-if-one-screen

[Copilot]

The quit_if_one_screen auto-exit in dynamic paging was triggered by two mechanisms: an explicit end_of_output() call and an implicit drop-based signal via AliveGuard. The implicit path added complexity and a subtle ref-count dance in dynamic_paging. Drop the implicit mechanism entirely; end_of_output() is the sole signal.

Changes

• src/pager.rs — Remove AliveGuard struct, its Drop impl, and the alive: Arc<AliveGuard> field from Pager. Simplify Clone and new(). Update set_quit_if_one_screen docs to document only end_of_output() as the end-of-stream signal.
• src/dynamic_pager.rs — Remove the workaround pager_for_init construction (which existed solely to hold an independent AliveGuard Arc). dynamic_paging now passes the pager directly to init_core.

Before

// Two paths to trigger CheckQuitIfOneScreen:
pager.end_of_output().unwrap();   // explicit
drop(pager);                       // implicit via AliveGuard::drop

After

// Only one path:
pager.end_of_output().unwrap();