[adm 3] Classify Heading authoring metadata

[artiphishle]

Package-owner release boundary for ankhorage/studio#118.

Context

The ADM roadmap revalidation found that Heading is already a direct manifest node with a useful blueprint, but the released ZORA v2.9.2 metadata does not classify its legitimate instance-authorable fields. The downstream Studio draft is ankhorage/studio#204 and must consume a released ZORA version rather than recreating this authority locally.

Image is intentionally not included: it remains directManifestNode: false and defers manifest media authoring to [adm 14].

What changed

• classify Heading text, i18nKey, semantic level, and numberOfLines as instance-authorable
• classify Heading size, color, emphasis, align, weight, and italic as component-theme authority
• add the previously missing serializable size metadata aligned with the public Heading API
• preserve the existing useful Heading blueprint defaults (text: "Heading", level: 2)
• extend authoring authority tests and documentation
• add a patch changeset for @ankhorage/zora

Boundaries

• ZORA owns the concrete Heading metadata and remains independent of Studio routes and UI.
• Semantic heading level remains per-node content semantics; visual presentation does not become a per-instance override.
• No Studio-specific schema, routing, editor, or persistence concern is added.
• No Image/media authoring is pulled forward from [adm 14].

Validation

Green on validation run 31059589165:

• frozen dependency install
• formatting and Paradox regeneration
• build
• Ankhorage Doctor validation
• lint and format check
• Knip
• full tests
• typecheck
• changeset status
• npm pack --dry-run

Release boundary

After this PR is merged and @ankhorage/zora is released, Studio PR #204 can update to the released package, add representative Heading integration coverage, rerun its full Web smoke/E2E validation, and complete [adm 3].