Add validation module

[DavidMStraub]

No description provided.

[Copilot]

Pull request overview

Adds a first-pass dataset validation facility to gedcom7, exposing a validate() API and integrating optional pre-serialization validation into dumps().

Changes:

• Introduces gedcom7.validate with an Error model and a validate(records) function to report multiple dataset issues in one pass.
• Adds GedcomValidationError and wires serializer.dumps(..., validate=True) to raise it with all discovered problems.
• Adds comprehensive tests covering dataset shape, pointers, payload kinds/values (incl. date sanity checks), schema declarations, and dumps validation behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/test_validate.py	Adds unit tests for the new validation behavior and dumps(validate=True) integration.
gedcom7/validate.py	Implements dataset validation (document shape, schema conflicts, payload checks, pointer checks, and date range sanity checks).
gedcom7/serializer.py	Adds validate option to dumps() and raises GedcomValidationError when validation fails.
gedcom7/exceptions.py	Introduces GedcomValidationError carrying all validation issues and summarizing them in the exception message.
gedcom7/init.py	Exports validate, Error, and GedcomValidationError as part of the public API.

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