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Milestone #67 step 3. Spell out, at the IngestCore.run seam resolution, that post_record_key + write_post_record are the post-first contract a platform implements when migrating onto the native core ingester — the post-record owns the body/links, the per-media sidecar carries image identity only. The import side is already self-enforcing via uses_native_ingester → importer.post_first. Durable directive recorded as FC project rule #120. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>