Add contextual editor markers and the context popup

[Azganoth]

Summary

Implement Leafdown's object-specific Markdown marker presentation and contextual editor popup behavior.

Expected behavior

• Marker presentation follows the documented per-object rules:
  • Headings, blockquotes, ordered lists, unordered lists, and task lists show subtle caret-based markers.
  • Horizontal rules, code blocks, and tables remain rendered visual objects with focused controls or affordances rather than raw delimiter editing.
  • Strong, emphasis, strikethrough, inline code, links, images, footnote references, autolinks, and raw HTML expose editable raw Markdown syntax when active.
  • Footnote definitions show a persistent subtle marker.
  • Paragraphs and hard breaks do not add custom marker UI.
• Selection alone does not reveal syntax markers.
• Focused code blocks edit code content directly and expose the language identifier as metadata/control, without turning the block into raw fenced Markdown.
• Selecting a rendered or placeholder image exposes the raw image Markdown for editing alt text and target path.
• The context popup opens from selection or right-click, preserves or moves the caret according to the click target, and exposes the documented MVP command groups.
• Escape, typing, outside clicks, or scrolling the popup out of view close it.

Related docs

• Decisions: Treat marker presentation as object specific
• Specification: Marker visibility state
• Specification: Marker visibility and presentation
• Specification: Blocks
• Specification: Inline content
• Specification: Context popup
• Specification: Context popup state
• Architecture: Leafdown responsibilities
• Decisions: Do not use crepe

Done when

• Marker presentation matches the documented per-object behavior.
• Selection alone does not reveal raw syntax markers.
• Code blocks, tables, and horizontal rules keep rendered object editing behavior rather than raw delimiter editing.
• Inline/source-oriented objects expose editable raw Markdown syntax when active.
• Footnote references and footnote definitions follow their separate marker rules.
• Image focus/selection exposes editable raw image Markdown.
• Selection and right-click popup opening rules preserve or reposition editor context as specified.
• Editor right-click handling uses Leafdown popup behavior rather than the default webview context menu.
• Popup close behavior and command grouping match the MVP context surface.
• Tests cover marker presentation transitions and popup context/close behavior; visual verification confirms the editor surface remains readable.