Scan report output can overwrite its selected suppression baseline #984

When a scan reads a suppression baseline and writes its report to the same path, the report replaces the baseline and the command exits successfully. Reusing that file loses the saved suppressions.

Reproduced on e14c450b260958ed3ebd83c94c09c4b093e3e656 with explicit JSON/SARIF output and the selected shipped baseline. A separate output path preserves the baseline.

Expected: reject a report destination that would overwrite the selected baseline before scanning, while preserving stdout, explicit baseline precedence, separate outputs and intentional baseline generation.