Add minimal editor power features without sacrificing performance

[trsdn]

Tracking issue:\n- [ ] https://github.com/trsdn/mdviewerplus/issues/12\n- [ ] https://github.com/trsdn/mdviewerplus/issues/13\n- [ ] https://github.com/trsdn/mdviewerplus/issues/14\n- [ ] https://github.com/trsdn/mdviewerplus/issues/15\n- [ ] https://github.com/trsdn/mdviewerplus/issues/16\n- [ ] https://github.com/trsdn/mdviewerplus/issues/17\n\nConstraints: no Mermaid, sidebar, polling, or parser migration; <=20 KB new vendored web assets; preserve native document lifecycle, unsaved edits, themes, scroll sync, printing, CSP, and sandboxing.\n\nInspired by the lightweight workflow ideas in the sdkks/mdviewer fork. Thanks to @sdkks for the implementation input. Attribution belongs in issue/release records, not README or GitHub Pages.

[trsdn]

The lightweight roadmap remains the Lite baseline. Edition architecture: #19. Full-only Mermaid/pan-zoom: #20. Full-only expanded highlighting: #21.

[trsdn]

Additional shared Lite/Full work: outline #23; GitHub alerts/task lists #24; image zoom #25; code-block controls #26. Full-only YAML frontmatter: #22.

[trsdn]

Implemented and released in MDViewer+ v2.0.0.