Escape-Hintable

Abstract marker class for AST nodes that may carry an `escapeHint` (added in 1.3.0). Two things are EscapeHintable: a StructuredEntry (the hint applies to the entry's value, the original 1.0.0 placement) and a StructuredValue (the hint travels with the value itself — needed for scalars that sit inside a StructuredList, where there is no entry to hold it). Modeled as a shared superclass rather than a multi-valued `escapeHint` domain because the protocol's domain checking is single-valued and resolves through the class hierarchy: one `domain: EscapeHintable` covers StructuredEntry and every StructuredValue subclass at once. When a hint is present in both places — on a StructuredEntry and on the StructuredValue it holds — the entry-level hint wins. It is the more specific, closer-to-the-author placement.