Summary

• Extend _validate() in Fd5Builder with JSON schema compliance checking, description quality warnings (non-blocking), and product-specific validation via duck-typed validate() method
• Add pre_seal_hooks parameter to create() for user-defined validation/inspection hooks called before sealing
• Add check_descriptions_file() to quality.py for checking description quality on open HDF5 files
• Make _walk() in quality.py resilient to external links that cannot be resolved

Test plan

• test_schema_validation_catches_missing_dataset -- schema requires 'values' group, not written, seal raises Fd5ValidationError
• test_schema_validation_passes_valid_file -- normal create + seal works
• test_description_quality_warns_short -- short description triggers UserWarning
• test_description_quality_does_not_block -- short description still seals successfully
• test_product_validate_method_called -- schema with custom validate() is called during seal
• test_product_validate_can_reject -- schema validate() raises, seal aborts
• test_pre_seal_hooks_called -- hook called with open h5py.File
• test_pre_seal_hook_can_abort -- hook raises, seal aborts, temp file cleaned up
• test_no_hooks_default -- default behavior unchanged
• All 57 tests in test_create.py + test_preseal_validation.py pass
• 1121 tests pass in full suite (pre-existing mip_coronal failures excluded)

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