Add locatorType field to citationMark schema

[gvonness-apolitical]

Summary

The citationMark schema in semantic.schema.json defines a locator field but has no corresponding locatorType field to indicate what the locator refers to (page, chapter, section, etc.).

Current schema

"locator": {
  "type": "string",
  "description": "Page, chapter, section, or other locator (CSL-compatible, more general than pages)"
}

Proposed addition

Add a locatorType property alongside locator:

"locatorType": {
  "type": "string",
  "description": "Type of locator (CSL-compatible). Defaults to \"page\" if omitted.",
  "enum": [
    "page", "chapter", "section", "paragraph", "line",
    "verse", "volume", "issue", "part", "book",
    "figure", "table", "note", "opus", "sub-verbo"
  ]
}

Rationale

• CSL compatibility: The Citation Style Language distinguishes locator (the value, e.g., "42-45") from locator-type (the kind, e.g., "page"). Without the type, renderers must guess or default to "page".
• Existing usage: The cdx-core reference implementation already emits locatorType in its citation_with_page() constructor. The field passes validation today only because additionalProperties: true, but it should be an explicit part of the schema.
• Renderer clarity: A citation like locator: "III" is ambiguous without knowing whether it refers to chapter III, volume III, or part III.

Impact

Non-breaking — this adds an optional property. Existing documents without locatorType remain valid.

[gvonness-apolitical]

Implemented in PR #31 (merged).

• Added locatorType enum to citationMark in semantic.schema.json with the full set of CSL-compatible locator types
• Updated the semantic-document example to use the structured form (locator: "230-265" + locatorType: "page")

The field is optional and defaults to "page" if omitted, so this is non-breaking for existing documents.