[Feature]: Add Go to Symbol for the current document

[h3pdesign]

Problem

The editor has outline-oriented surfaces, but it still lacks a direct symbol jump workflow for quickly navigating larger files.

Proposed solution

Add a lightweight Go to Symbol command for the current document, using the existing document structure model where possible.

Acceptance criteria

• Users can open Go to Symbol from the command surface
• Symbols are searchable and jump to the correct location
• The feature works well with keyboard-only navigation
• VoiceOver users can identify entries clearly
• The implementation stays lightweight and document-scoped