Fix fractional wheel scrolling in scrollback and alternate screen

[jerezereh]

Addresses issues #781 (commit 30738db) and #385 (commit 0468347).

Wheel scrolling previously cast float inputs to ints, dropping all fractional scrolling that occurs on most modern, high-resolution scroll wheels. Commit 30738db addresses this by keeping scroll delta as float, and saving remainders to state.

Additionally noticed while testing that scroll events were not received in ALT_SCREEN mode (less, vi, man, etc.). Fixes in commit 0468347 address this by allowing non-SGR mouse mode wheel events, sending mouse wheel or arrow key events depending on TermMode::APP_CURSOR, and saving the fractional mouse wheel events (as in the previous commit).

Tests have been added to both terminal_box.rs and mouse_reporter.rs.

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Disclaimer: As I am new to Rust, AI agent Codex was used for reference and to review code. If any changes here don't look right, I would be happy to receive feedback or guidance.