Fix markdown TOC navigation and selection gestures

[loybot]

Summary

• make rendered Vditor TOC entries scroll to headings in preview, split, and editor views
• route hash link scrolling through the nearest scroll container
• ignore and restore selection-only Vditor input that can collapse single line breaks during double-click selection

Verification

• npm run build
• npx --yes @haloforge/plugin-pack@0.2.15 check .
• npx --yes @haloforge/plugin-pack@0.2.15 pack . --release --out dist/plugin-release
• npm run hf -- plugin install local /Users/h3ddev/Halo/hf-plugin-markdown/dist/plugin-release/dev.haloforge.markdown-0.2.10.hfpkg --json