chore: every OS checks out LF (P11c)

The Windows job of check.yml gets CRLF from the runner's autocrlf, and the
golden, fixture and hash-pinned files are compared byte for byte. No file
in the index has CRLF, so nothing changes on Linux or macOS.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

.gitattributes

@@ -0,0 +1,3 @@
+# Tests compare text and checked-in hashes byte for byte: every OS checks out LF (the
+# Windows job of check.yml would otherwise get CRLF from autocrlf).
+* text=auto eol=lf