Post-launch P1: improve export diagnostics and unsupported-rule hints

[honor2030]

Goal

Make exports more self-explanatory so support load stays low as more formats are added.

Acceptance criteria

• Users can see format-specific warnings before download.
• Blocked or modified rows are explained clearly.
• The export screen links to short format guidance.

Task list

• Add per-format warning copy to each export adapter.
• Add row-level explanation handling.
• Link to lightweight format docs (Shopify, Netlify, Cloudflare, Vercel).
• Update manual QA checklist.

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Estimate: 0.5 day