feat(tui): improve diff span rendering

[CSRessel]

Summary

• Ports the upstream Codex diff-rendering approach into Nori's smaller TUI surface.
• Preserves syntax-highlighter state across update hunks and wraps styled spans by terminal display width.
• Uses move/update destination paths for syntax detection and documents the diff-rendering invariant.

Test Plan

• cargo test -p nori-tui diff_render
• cargo test -p nori-tui
• cargo build --bin nori
• cargo build --bin mock_acp_agent
• cargo test -p tui-pty-e2e
• just fmt
• just fix -p nori-tui

Generated with Nori.