feat: bear-style text editor parity

[luctst]

Summary

Enrich the Marcdown text editor to feel as close to Bear's editor as possible — the goal being a daily-driver markdown editor good enough to replace Raycast Notes.

Built via a 4-agent workflow: PM functional spec → Thomas design review → oonolive-swift tests-first → guy implementation.

What lands

Styling helpers (each as its own commit):

• Depth-aware MarcdownListMarkerKind (bullets cycle filled/hollow circle → filled/hollow diamond by depth; ordered carries its depth)
• ListIndentation — Tab / Shift+Tab indent/outdent on list lines (2-space step)
• OrderedListRenumber — renumbers contiguous ordered run at the anchor's depth, skipping deeper nested children
• BlockquoteContinuation — Enter continues > ; Enter on empty > exits (strips one level for nested)
• HeadingContinuation — Backspace on empty # strips the marker atomically
• FocusLine + MarkdownStyler.restyle(storage:source:focusLine:) — concealed syntax (**, link ](url), list markers, etc.) reveals on the line the cursor is on, re-conceals on leave
• ListContinuation refinements — empty-marker exit emits blank-line separator after content; backspace at marker-start triggers atomic join

Editor wiring (NoteEditorView.Coordinator):

• Routes insertTab:, insertBacktab:, insertNewlineIgnoringFieldEditor:, moveLeft:, moveRight: to the new helpers
• Arrow keys jump over concealed runs (link URLs, bold delimiters, etc.) in a single keystroke
• Runs OrderedListRenumber after every list-affecting edit
• Recomputes FocusLine on selection change and feeds it to restyle
• CheckboxIconLayoutManager draws depth-aware bullet glyphs and uses body color for both bullets and ordered numbers

Test plan

Automated (all green locally):

• swift test --package-path Packages/MarcdownStyling — 293/293 passing
• swift test --package-path Packages/MarcdownEditor — 31/31 passing
• swift test --package-path Packages/MarcdownCore — 27/27 passing
• xcodebuild -scheme Marcdown -configuration Debug build — SUCCEEDED

Manual (please verify in the running app):

• Cursor entering a line with **bold** / [label](url) / list markers reveals the syntax, leaves re-conceal it
• Tab/Shift+Tab on bullet / ordered / task lines indents/outdents
• Enter on > hello continues the blockquote; Enter on empty > exits
• Backspace on empty # strips the marker
• Backspace at start of - bar (after - foo) joins lines and strips the marker
• Empty-bullet Enter after content inserts a blank-line separator
• Left/Right arrow jumps concealed runs in one keystroke
• Nested bullets render with the depth-aware glyph cycle (●, ○, ◆, ◇)
• Ordered numbers render in body color, not accent
• IME composition still works on every new path

Deferred (out of scope for this PR)

• Multi-line selection Tab/Shift+Tab (caret-only handled today)
• Cmd+B / Cmd+I formatting shortcuts (separate workstream)
• Focus-reveal animation / debounce — currently instant
• Move-line up/down (Cmd+Opt+Up/Down)
• Drag-to-reorder list items
• Checkbox toggle bounce / strikethrough wipe animations

Known cost

OrderedListRenumber.renumberRun returns a full-buffer rewrite that the coordinator applies as a single replaceCharacters — each renumber is one fat undo step. Worth optimizing later to (range, replacement) for just the changed sub-run.