fix(tooling): keep license headers idempotent on CRLF

[Gujiassh]

Summary

• recognize canonical license headers with either LF or CRLF line endings
• leave already licensed files byte-identical instead of prepending a duplicate header
• preserve the existing shebang exemptions and missing-header behavior
• add hermetic Node tests that run only against temporary copied repositories

Fixes #1171

Testing

• corepack pnpm@10.6.5 test on Node 18.20.8 and Node 22 (3/3)
• independent 13-fixture, two-run base/head differential smoke
• node --check for the implementation and tests
• normal repository commit hooks, including the license writer and Rush check
• targeted formatting and git diff --check