Add a native document outline popover without a persistent sidebar

[trsdn]

Goal

Provide fast heading navigation in both Lite and Full without adding a persistent sidebar, background index, or web dependency.

Requirements

• Expose the outline as a native toolbar/menu popover with keyboard navigation and search-as-you-type.
• Derive entries from Markdown headings and preserve heading hierarchy.
• Use exactly the same duplicate-heading slug rules as rendered anchors.
• Selecting an entry scrolls the preview and moves the editor caret to the corresponding source heading when appropriate.
• Refresh incrementally during edits without degrading typing or preview debounce.
• Handle empty, duplicate, Unicode, and very large heading sets deterministically.
• Add no third-party dependency and no persistent folder index.

[trsdn]

Implemented and released in MDViewer+ v2.0.0.