Pages QA: validate local URL fragments

[Codewriter90x]

scripts/validate_site.py verifies that local files exist, but it does not yet verify that #fragment targets exist in the destination HTML.

Acceptance criteria:

• parse fragments for local links;
• verify the destination page contains the referenced ID;
• add one positive and one negative unit test;
• keep the validator dependency-free and deterministic;
• run Ruff, mypy and the Pages tests.

This task does not modify generated dataset files.