Implement export adapters: Generic CSV and Netlify _redirects

[honor2030]

Goal

Give users the first two downloadable export formats.

Acceptance criteria

• Generic CSV exports parse correctly in Excel and Numbers.
• Netlify _redirects syntax is valid for a known-good sample.
• Rows with hard validation errors are excluded from all exports.
• Downloads trigger correctly without a server round-trip.

Task list

• Build Generic CSV adapter (source, destination, status-code columns).
• Build Netlify _redirects adapter.
• Add error-row filtering to both adapters.
• Test downloads in Chrome and Firefox.

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Estimate: 0.5 day