Composition templates place group with a bar or ladder arrangement

Acceptance Criteria

• A template can place {% group %} with a required name and layout of bar or ladder; it renders with data-name and no data-rune, and an unknown layout is rejected at schema construction, naming it (D2)
• A bar group renders data-zone-layout="bar" over what it holds, and a child written as {% group name="…" align="end" %} renders data-align="end" and pushes to the end (D2)
• A ladder group renders <ol> when the definition declares a sequence and <ul> otherwise, with data-zone-layout="ladder", and wraps each each iteration placed directly in it, or else each top-level node it holds, in an <li> (D3)
• Skeleton gains [data-zone-layout="ladder"]: the list reset and column flow for a ladder with no sequence. cast's list mode uses it and its own reset is deleted (D3)
• {% group %} is a template tag only: on a page it is reported as an unknown tag, as today (D2)
• A group is safe where schema is involved: a node-sourced schema property inside a group resolves, asserted against a placed {% bar %} where it does not (D9)
• The composed-runes guide documents {% group %} and D9's grouping rule
• Contracts are regenerated on both copies and the CSS coverage test passes