spec: print render language — language / languageFrom on snapshot children and notify attachments

[delchev]

Specifies the render-language knobs added to the reference implementation:

• A function: Snapshot child may declare language: <code> (fixed) or languageFrom: <relation>.<field> (per record - a one-hop path on the document master whose string field holds the language code; may cross model boundaries). Mutually exclusive; absent both, the mint falls back to the first entry of the application's configured language set at mint time; an unresolvable path is a generation error, never a silent wrong-language copy. Interactive printing renders live in the user-picked language; the snapshot panel serves the frozen issued copies - the two coexist.
• The notify block's attach: print gains the same languageFrom next to its existing language:, with the same fallback.

(The .numbers partition-source declaration touches the numbering section that #2 rewrites - noted there rather than conflicting here.)