Outline navigation for long documents

[tosinamuda]

User story
As a thesis writer in a 40-page chapter, I want an outline of my document's headings that I can click to jump, so that navigating long documents stops being scrolling archaeology.

Acceptance criteria

• An outline surface lists the active document's headings (H1–H4, nested)
• Click scrolls the editor to that heading (honest widget heights — fix(editor): scroll jump on select after fast scroll — honest widget height estimates #120 — so jumps land exactly)
• The current section highlights as I scroll and updates live as I type/restructure
• Works in rich and raw modes, and for external files (Compose as default markdown opener — with a deliberate landing policy (Drafts vs in-place) #113)
• Zero editor-perf regression: outline extraction rides the existing parse, no extra full-doc scans per keystroke

Sub-tasks

• Heading extraction from the Lezer tree (debounced, incremental)
• UI surface (sidebar tab vs popover — pick one, note why)
• Scroll-position → active-section tracking
• Tests (extraction contract + WebKit click-jump)