Git: mark PDFs, keys, fonts and images binary so Windows checkouts keep their bytes

CI's Windows runner checks out with core.autocrlf=true, which rewrote the
signed openssl-signed.pdf fixture and made validates_a_signature_made_by_openssl
report Invalid there.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,9 @@
+# Byte-exact fixtures and assets: never convert line endings (Windows checkouts use
+# core.autocrlf=true, which changes the signed bytes of a PDF and breaks its signature).
+*.pdf binary
+*.p12 binary
+*.pem -text
+*.pfb binary
+*.ttf binary
+*.icc binary
+*.png binary