Reject repeated declarations before version or changelog generation #481

🤖 Generated by the Agentic Engineer

Problem

Repeated JSON fields can hide marketplace entries or version changes before release helpers validate them. A generated version bump can erase an earlier declaration, and a changelog check can miss required notes.

Proposal

Validate unique decoded declaration paths before parsing or staging marketplace and plugin manifests.

Acceptance criteria

Repeated or escaped-equivalent scalar and container fields refuse generation/checking with original bytes preserved; valid paired manifests and normal version bumps still pass.

Reproduced offline against freshly fetched main using synthetic fixtures. No inspected source was executed and no provider experiment was launched. Size: S.

Part of #101.