Port 'Expose Settings.xml Visibility' from machine

[Copilot]

Ports changes from sillsdev/machine#412 — exposes the <Visibility> field from Paratext Settings.xml.

Changes

• ParatextProjectSettings — Added visibility: Optional[str] = None field
• ParatextProjectSettingsParserBase — Reads <Visibility> element and passes it to the constructor
• Tests — Added test_parse() to both file and zip parser tests asserting all settings fields; updated _create_settings() helper to include "Public" for visibility

parser = FileParatextProjectSettingsParser(project_path)
settings = parser.parse()
assert settings.visibility == "Public"  # read from <Visibility>Public</Visibility> in Settings.xml

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