Support clearing scrollback

[donbeave]

Summary

This adds a narrow scrollback-clearing path to vt100:

• Screen::clear_scrollback() clears the active screen's saved scrollback and returns the scrollback view to the live viewport.
• CSI 3 J is handled as xterm's "erase saved lines" operation.
• The visible grid is left intact, and terminal modes are not reset.

I also included two small CI hygiene updates so this PR can run cleanly against the current workflow: the existing example loop is adjusted for current clippy, and the lockfile updates the dev-only rand entry from 0.9.1 to 0.9.4 to clear the current RustSec advisory reported by cargo deny.

Motivation

vt100 already models the terminal screen, scrollback, and parser state for consumers building terminal wrappers/multiplexers. Those consumers need to distinguish "clear saved lines" from heavier reset-style behavior:

• CSI 3 J should clear scrollback.
• It should not erase the visible screen.
• It should not reset modes such as application cursor, cursor visibility, mouse reporting, or bracketed paste.

That makes this fit vt100's existing responsibility better than forcing each downstream consumer to reach into local state or fake a terminal reset.

This is intentionally narrower than #22. That PR proposed broader scrollback inspection APIs. This PR only adds the missing clear operation and terminal escape handling, without exposing scrollback rows or internal row types.

Tests

Added coverage in tests/scroll.rs for:

• direct Screen::clear_scrollback() behavior;
• CSI 3 J clearing saved lines;
• preserving visible contents after CSI 3 J;
• clamping/resetting the scrollback offset after clear;
• preserving application cursor, hidden cursor, mouse reporting, SGR mouse encoding, and bracketed paste after CSI 3 J.

Local verification

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• cargo build --all-targets
• cargo doc --no-deps
• rustup run 1.70.0 cargo check --lib --all-features
• cargo deny check