v0.1.0

Removed

Breaking. Four names left the public API. All four were implementation
details that no documented API returned or accepted, and nothing in the
example or the docs used them:

• ControlSizeResolver — the internal ControlSize.resolve1D helper.
• SpinButton — chrome internal to InputNumber.
• detectBorderRadiusFromContext, detectBorderRadiusFromWidget.

Everything else the kit exported stays: types such as PopoverPlacement and
RailInsets appear in public signatures (Tooltip.placement,
TimelineToken.railInset), so callers need to be able to name them.

Changed

• Breaking. Timeline.items now takes List<TimelineEntry>.
  TimelineEntry is sealed over exactly two cases, TimelineItem and
  TimelineGroupItem. Lists of plain items keep working unchanged.

  TimelineGroupItem used to extend TimelineItem, which gave it fourteen
  inherited fields it never read — a caller reaching for color or dot on a
  group got silence — and let a group nest inside another group, which
  compiled but drew an empty node. Neither is expressible now.

Fixed

• Tour eased its panel into place but dropped the mask on at full strength,
  so opening a tour read as a flash. The dim now fades in over the theme's mid
  duration, matching the popover barrier.

Added

• A snapshot test over the exported API (test/public_api_test.dart). Any
  change to lib/seed_ui.dart's surface now shows up as a reviewable diff,
  and a bare export without a show clause fails the suite.
• CONTRIBUTING.md, issue forms and a pull-request template.
• Tests for TimelineGroupController, collapsible timeline groups, and the
  horizontal and reversed timeline layouts.