The Casting Syntax

Every form the ui! { … } macro accepts. Six axes:

• Context Area — the :( … :) header that opens every cast.
• Widget Nodes — the heart of the language: named or positional attrs, optional parens, optional body.
• Enchants — the [expr, expr, …] block that attaches arbitrary data to a widget.
• Control Flow — if, walk … with …, @niche, match.
• The on Handlers — event clauses in two forms (B and C), body or callback, qualified or bare, with or without args.
• Rejected Forms — what the parser refuses, and why.

The DSL is untyped at parse time. Widget names are arbitrary identifiers; attribute values are arbitrary syn::Expr. All semantics — what counts as a widget, which attrs are valid, what an event kind means — live in the rune you bind, never in xrune itself.