Onya Literate: support multiple types per node header (writer emits them, parser rejects them)

[uogbuji]

The Onya model allows a node to have a set of types, and the Literate writer already serializes multiple types space-separated, but the
parser cannot read them back, so any graph with a multi-typed node fails to round-trip, and hand-authored multi-type headers crash with a confusing error.

Repro

# @docheader
* @document: https://example.org/g/doc
* @nodebase: https://example.org/g/
* @schema: https://schema.org/
* @iri:
    * lv: https://example.org/g/schema

# acme [Organization lv:Client]
* name: ACME Corp

onya convert file.onya.md --mermaid →

NameError: name '_' is not defined

Root cause

1. Writer (literate.py): type_str = ' '.join(type_parts) emits all types,
   space-separated—e.g. # acme [Organization lv:Client].
2. Parser (_literate_parse.py): the header grammar captures the bracket
   content as a single token
   (Optional(QuotedString('[', end_quote_char=']'))), and
   process_nodeblock (~line 435) expands that whole string as one IRI:
   expand_iri("Organization lv:Client", …). The space makes it an invalid IRI.
3. The invalid-IRI branch in amara/iri/irihelper.py (~line 59) raises
   ValueError(_(f'Invalid IRI reference: …')), but _ (a gettext alias) is
   undefined in that module → the real error is masked by NameError: name '_' is not defined. (Secondary bug worth fixing independently — it makes every
   invalid-IRI failure opaque.)

Ergo a node with two types serializes fine but cannot be parsed. The round-trip
parse → write → parse is broken for any multi-typed node.

Proposed resolution

• SPEC (#onya-literate-serialization): define the multi-type header syntax
  explicitly — recommend space-separated types inside […]
  ([Organization lv:Client]), matching what the writer already emits, and
  state that types are a set.
• Parser: tokenize the bracket content into 1..N type refs (split on
  whitespace, respecting <…> CURIE/IRI wrappers), expand each, and add all to
  node.types.
• Writer: already correct; add a round-trip test over a 2-type node.

Note: this upstream wart exacerbated the diagnosis of this problem.