Add .gitattributes to normalise line endings

Fixes Windows git showing modified files after Linux commits.
LF stored in repo; CRLF on Windows checkout for all source files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,19 @@
+# Normalise line endings: LF in repo, native on checkout
+* text=auto
+
+# Force CRLF on checkout for Windows source files
+*.cs    text eol=crlf
+*.xaml  text eol=crlf
+*.csproj text eol=crlf
+*.sln   text eol=crlf
+*.json  text eol=crlf
+*.txt   text eol=crlf
+*.md    text eol=crlf
+*.csv   text eol=crlf
+
+# Binaries — no line-ending conversion
+*.png   binary
+*.jpg   binary
+*.ico   binary
+*.dll   binary
+*.exe   binary