file_header

The first N lines must match a regex (line-oriented). For a byte-level prefix check, prefer file_starts_with.

- id: spdx-header
  kind: header
  paths: "src/**/*.rs"
  pattern: "^// SPDX-License-Identifier: MIT"
  level: error

Fix: file_prepend — inject declared content at the top (preserves UTF-8 BOM).