fix(tui): coalesce session scrollback input

[edwin-zvs]

Summary

• coalesce queued mouse wheel events before rendering so fast scrolling does not replay one event at a time after input stops
• make mouse wheel scrollback use a partial-page step based on terminal height instead of a tiny fixed row step
• clamp the modeline scrollback value to the effective rendered vt100 scrollback so it cannot drift past available history

Tests

• cargo test -p agentd-cli scrollback -- --nocapture
• cargo test -p agentd-cli drain -- --nocapture
• cargo test -p agentd-cli
• cargo build
• git diff --check