Implement redirect validation engine

[honor2030]

Goal

Catch format-level and logic-level problems before the user downloads an export.

Acceptance criteria

• Duplicate source URLs produce a visible error.
• Self-redirects (source = destination) are flagged as errors.
• Chains (A→B and B→C) produce a warning.
• Validated results feed the export adapters.

Task list

• Implement duplicate source detector.
• Implement self-redirect checker.
• Implement chain/loop detector.
• Surface results in the review table and export screen.

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Estimate: 0.5 day