Improve desktop text buffer reuse on resize

Resize benchmark p95 improved from 13.37ms to 12.85ms for optimized cached visible body path; remains under 16ms target.\n\nValidation: cargo check -p jcode-desktop; cargo test -p jcode-desktop; selfdev desktop build; target/selfdev/jcode-desktop --resize-render-benchmark=60; target/selfdev/jcode-desktop --scroll-render-benchmark=60.