Function build_dry_run_envelope 

pub fn build_dry_run_envelope(ref_: &Ref, plan: &FetchPlan) -> Value

Build the dry-run envelope as a serde_json::Value, without writing anywhere. Used by both the CLI (which prints it to stdout) and the MCP tool wrapper (which routes the bytes via JSON-RPC). Wire shape:

{
  "ok": true,
  "dry_run": true,
  "ref": { "doi": "10.1234/foo" } | { "arxiv": "2401.12345" },
  "plan": { ... see FetchPlan ... },
  "rate_limit_budget": { "global_per_sec": 5.0, "per_source_min_gap_ms": 200 }
}