Add .gitattributes marking PDFs and C64 images as binary

vim-user-guide.pdf has no NUL bytes, so git auto-detected it as text and
core.autocrlf rewrote its line endings on checkout, corrupting it. Mark
PDF, PNG, PRG, BIN, and D64/D71/D81 files binary so they are never converted.

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

.gitattributes

@@ -0,0 +1,9 @@
+# Never apply line-ending conversion to binary files. A PDF with no NUL bytes
+# is otherwise auto-detected as text and corrupted by core.autocrlf.
+*.pdf binary
+*.png binary
+*.prg binary
+*.bin binary
+*.d64 binary
+*.d71 binary
+*.d81 binary