Additive Semantics (v2)

v2 is additive and backward compatible with v1: it introduces semantics and viewer attributes on top of the Markdown + Components grammar. No grammar changes in v2. New attributes/viewers are validated downstream. Key additions:

• artifact.json: json_pointer (RFC 6901) for static projection; depth bound; collapsed flag.
• artifact.table: columns (keys or JSON Pointers); optional caption; optional kind hint for schema.
• artifact.image: optional caption; alt remains required; max_height bounds.
• artifact.text: new viewer with max_lines=1..500 and optional caption.

Validation and rendering layers enforce bounds, unknown attrs, and advisory schema hints.