Implement migration artifact taxonomy and accountable import/export receipts

[mdheller]

Purpose

Apply SourceOS Mutation and Evidence Accountability to Exodus migration/import/export flows.

Canonical spec PR:

SourceOS-Linux/sourceos-spec#96

Required artifact taxonomy

Exodus must distinguish:

• durable user-owned media;
• durable documents;
• message attachments;
• derived previews;
• codec temporary files;
• thumbnails;
• attachment caches;
• sync staging;
• failed transcode residue;
• opaque vendor state;
• archive extraction temp files;
• diagnostic/export artifacts.

Required receipts

• ArchiveExtractionReceipt
• TemporaryArtifactLifecycleReceipt
• MediaWorkReceipt
• MutationReceipt
• EvidencePipelineReceipt
• ownership/attestation receipts for user-owned imports

Acceptance criteria

• Migration manifests explain imported, excluded, quarantined, reconstructed, and ignored artifacts.
• Vendor caches and codec temps are not imported as durable user content by default.
• Archive extraction includes source archive hash/class, output path class, file count, byte count, permission/xattr changes, quarantine state, cleanup state, and downstream indexing/sync triggers.
• Ownership/attestation evidence is captured without acting as media police beyond marketplace/distribution boundaries.