Configurable bullet shapes and task checkbox style

[luisKisters]

Bullet markers and task checkboxes are the only list rendering an embedder cannot influence today: drawBulletMarkers paints a hardcoded "•" in theme.bodyText at every nesting depth, and drawTaskCheckboxes paints the SF Symbols square / checkmark.square.fill. ListStyle carries indent and line-height knobs but nothing for the markers themselves.

This adds two configuration groups so a host app can restyle both without forking.

API

config.lists = ListStyle(
    bullets: BulletStyle(
        shapeLadder: [.filledDot, .hollowRing, .smallSquare, .triangle],
        color: .systemPurple
    ),
    taskCheckbox: TaskCheckboxStyle(
        rendering: .drawn,
        size: 15, strokeWidth: 1.5, cornerRadius: 4.5, gap: 6,
        uncheckedColor: .systemPurple,
        checkedFillColor: .systemPurple
    )
)

BulletStyle.shapeLadder is indexed by 1-based nesting depth; the last entry repeats for deeper levels, and an empty ladder falls back to .filledDot. Shapes other than .filledDot are NSBezierPath, so they stay crisp at any scale.

Defaults are unchanged

BulletStyle.default is [.filledDot], which keeps the exact existing "•" NSString draw — same font, same centring within the marker advance. TaskCheckboxStyle.default is .systemSymbol, which keeps the existing SF Symbol branch untouched. All 253 existing tests pass with no edits to them.

rendering is an explicit mode rather than an inferred self == .default, so adding a field later cannot silently re-route existing embedders between the two paths.

How depth reaches the renderer

MarkdownASTStyler.styleListItem already computes MarkdownLists.indentLevel(from:) for its indent. It now also writes that depth (1-based) onto the marker range as a bulletListLevel attribute, following the existing blockquoteLevel precedent. drawBulletMarkers reads it and defaults to depth 1 when absent. Both attributes are written together and cleared together by the existing whole-paragraph setAttributes, so a stale depth cannot survive an edit.

Draw / hit-test parity

TaskCheckboxGeometry stays the single source of truth. size(for:style:) and boxX(contentX:size:gap:) are used by both MarkdownTextLayoutFragment.drawTaskCheckboxes and NativeTextView+TaskCheckbox.taskCheckboxHit, so a configured size or gap moves the drawn box and the click target together.

Tests

Added: default-preservation for both styles, ladder clamping, the 0-based indentLevel to 1-based ladder conversion across a real nested list, and the checkbox size and gap geometry. 255 tests pass.

One thing worth your call: MarkdownLists.indentLevel(from:) is a whitespace heuristic (tabs plus spaces / 2) that does not consult lists.indentPerLevel, so 4-space-indented CommonMark resolves one level deeper than it looks. That is pre-existing behaviour and this PR does not touch it, but the shape ladder makes it visible for the first time.