Preserve mixed evaluation evidence with reserved atomic report checkpoints

tools/mixed-eval/README.md

@@ -36,3 +36,5 @@ The raw report retains queries, expected IDs, ordered hits, source locations,
 scores, modes/scopes/languages and MCP roundtrip times. Those times exclude
 process/model initialization and do not represent native UI latency. Keep raw
 reports under ignored `.searchi`; review deliberate evidence before publishing.
+Use a new output filename: existing evidence is rejected before inference.
+Checkpoints replace JSON atomically so a failed write preserves the prior report.